apacheGH-40249: [Java] Fix NPE in ArrowDatabaseMetadata

### Rationale for this change

When retrieving database metadata using the JDBC driver, some data such as SQL keywords
could be null. Before this change, an NPE would be thrown when trying to convert the
list of SQL keywords into a String.

### What changes are included in this PR?

The following database metadata fields:

* SQL keywords
* Numeric functions
* String functions
* System functions
* Time/date functions

will convert to an empty string when they are null.

### Are these changes tested?

A unit test has been added to verify that the fields above are converted to the empty
string when null, with no exceptions thrown.

### Are there any user-facing changes?

The fields above will now return an empty string rather than throw an NPE.

java/flight/flight-sql-jdbc-core/src/main/java/org/apache/arrow/driver/jdbc/ArrowDatabaseMetadata.java

@@ -49,6 +49,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
@@ -197,31 +198,36 @@ public boolean isReadOnly() throws SQLException {
   @Override
   public String getSQLKeywords() throws SQLException {
     return convertListSqlInfoToString(
-        getSqlInfoAndCacheIfCacheIsEmpty(SqlInfo.SQL_KEYWORDS, List.class));
+        getSqlInfoAndCacheIfCacheIsEmpty(SqlInfo.SQL_KEYWORDS, List.class))
+        .orElse("");
   }
 
   @Override
   public String getNumericFunctions() throws SQLException {
     return convertListSqlInfoToString(
-        getSqlInfoAndCacheIfCacheIsEmpty(SqlInfo.SQL_NUMERIC_FUNCTIONS, List.class));
+        getSqlInfoAndCacheIfCacheIsEmpty(SqlInfo.SQL_NUMERIC_FUNCTIONS, List.class))
+        .orElse("");
   }
 
   @Override
   public String getStringFunctions() throws SQLException {
     return convertListSqlInfoToString(
-        getSqlInfoAndCacheIfCacheIsEmpty(SqlInfo.SQL_STRING_FUNCTIONS, List.class));
+        getSqlInfoAndCacheIfCacheIsEmpty(SqlInfo.SQL_STRING_FUNCTIONS, List.class))
+        .orElse("");
   }
 
   @Override
   public String getSystemFunctions() throws SQLException {
     return convertListSqlInfoToString(
-        getSqlInfoAndCacheIfCacheIsEmpty(SqlInfo.SQL_SYSTEM_FUNCTIONS, List.class));
+        getSqlInfoAndCacheIfCacheIsEmpty(SqlInfo.SQL_SYSTEM_FUNCTIONS, List.class))
+        .orElse("");
   }
 
   @Override
   public String getTimeDateFunctions() throws SQLException {
     return convertListSqlInfoToString(
-        getSqlInfoAndCacheIfCacheIsEmpty(SqlInfo.SQL_DATETIME_FUNCTIONS, List.class));
+        getSqlInfoAndCacheIfCacheIsEmpty(SqlInfo.SQL_DATETIME_FUNCTIONS, List.class))
+        .orElse("");
   }
 
   @Override
@@ -753,8 +759,12 @@ private <T> T getSqlInfoAndCacheIfCacheIsEmpty(final SqlInfo sqlInfoCommand,
     return desiredType.cast(cachedSqlInfo.get(sqlInfoCommand));
   }
 
-  private String convertListSqlInfoToString(final List<?> sqlInfoList) {
-    return sqlInfoList.stream().map(Object::toString).collect(Collectors.joining(", "));
+  private Optional<String> convertListSqlInfoToString(final List<?> sqlInfoList) {
+    if (sqlInfoList == null) {
+      return Optional.empty();
+    } else {
+      return Optional.of(sqlInfoList.stream().map(Object::toString).collect(Collectors.joining(", ")));
+    }
   }
 
   private boolean getSqlInfoEnumOptionAndCacheIfCacheIsEmpty(

java/flight/flight-sql-jdbc-core/src/test/java/org/apache/arrow/driver/jdbc/ArrowDatabaseMetadataTest.java

@@ -95,9 +95,14 @@
 public class ArrowDatabaseMetadataTest {
   public static final boolean EXPECTED_MAX_ROW_SIZE_INCLUDES_BLOBS = false;
   private static final MockFlightSqlProducer FLIGHT_SQL_PRODUCER = new MockFlightSqlProducer();
+  private static final MockFlightSqlProducer FLIGHT_SQL_PRODUCER_EMPTY_SQLINFO =
+      new MockFlightSqlProducer();
   @ClassRule
   public static final FlightServerTestRule FLIGHT_SERVER_TEST_RULE = FlightServerTestRule
       .createStandardTestRule(FLIGHT_SQL_PRODUCER);
+  @ClassRule
+  public static final FlightServerTestRule FLIGHT_SERVER_EMPTY_SQLINFO_TEST_RULE =
+      FlightServerTestRule.createStandardTestRule(FLIGHT_SQL_PRODUCER_EMPTY_SQLINFO);
   private static final int ROW_COUNT = 10;
   private static final List<List<Object>> EXPECTED_GET_CATALOGS_RESULTS =
       range(0, ROW_COUNT)
@@ -604,7 +609,7 @@ public static void setUpBeforeClass() throws SQLException {
 
   @AfterClass
   public static void tearDown() throws Exception {
-    AutoCloseables.close(connection, FLIGHT_SQL_PRODUCER);
+    AutoCloseables.close(connection, FLIGHT_SQL_PRODUCER, FLIGHT_SQL_PRODUCER_EMPTY_SQLINFO);
   }
 
 
@@ -1420,4 +1425,16 @@ public void testSqlToRegexLike() {
     Assert.assertEquals("\\*", ArrowDatabaseMetadata.sqlToRegexLike("*"));
     Assert.assertEquals("T\\*E.S.*T", ArrowDatabaseMetadata.sqlToRegexLike("T*E_S%T"));
   }
+
+  @Test
+  public void testEmptySqlInfo() throws Exception {
+    try (final Connection testConnection = FLIGHT_SERVER_EMPTY_SQLINFO_TEST_RULE.getConnection(false)) {
+      final DatabaseMetaData metaData = testConnection.getMetaData();
+      collector.checkThat(metaData.getSQLKeywords(), is(""));
+      collector.checkThat(metaData.getNumericFunctions(), is(""));
+      collector.checkThat(metaData.getStringFunctions(), is(""));
+      collector.checkThat(metaData.getSystemFunctions(), is(""));
+      collector.checkThat(metaData.getTimeDateFunctions(), is(""));
+    }
+  }
 }