Add restrictions while setting search field attribute (#3124)

- Attribute cannot be null
- Attribute cannot be set multiple times

src/main/java/redis/clients/jedis/search/FieldName.java

@@ -25,6 +25,12 @@ public FieldName(String name, String attribute) {
   }
 
   public FieldName as(String attribute) {
+    if (attribute == null) {
+      throw new IllegalArgumentException("Setting null as field attribute is not allowed.");
+    }
+    if (this.attribute != null) {
+      throw new IllegalStateException("Attribute for this field is already set.");
+    }
     this.attribute = attribute;
     return this;
   }

src/test/java/redis/clients/jedis/modules/search/SchemaTest.java

@@ -1,12 +1,14 @@
 package redis.clients.jedis.modules.search;
 
+import static org.junit.Assert.assertThrows;
+
+import java.util.Collections;
 import org.hamcrest.CoreMatchers;
 import org.hamcrest.MatcherAssert;
 import org.junit.Test;
+import redis.clients.jedis.search.FieldName;
 import redis.clients.jedis.search.Schema;
 
-import java.util.Collections;
-
 public class SchemaTest {
 
   private final static String TITLE = "title";
@@ -33,4 +35,15 @@ public void printSchemaTest() throws Exception {
     MatcherAssert.assertThat(schemaPrint, CoreMatchers.containsString("{name='release_year', type=NUMERIC, sortable=true, noindex=false}"));
     MatcherAssert.assertThat(schemaPrint, CoreMatchers.containsString("VectorField{name='vector', type=VECTOR, algorithm=HNSW"));
   }
+
+  @Test
+  public void fieldAttributeNull() {
+    assertThrows(IllegalArgumentException.class, () -> FieldName.of("identifier").as(null));
+  }
+
+  @Test
+  public void fieldAttributeMultiple() {
+    assertThrows(IllegalStateException.class, () -> FieldName.of("identifier").as("attribute").as("attribute"));
+    assertThrows(IllegalStateException.class, () -> new FieldName("identifier", "attribute").as("attribute"));
+  }
 }