Fixed StringIndexOutOfBoundsException caused by empty index expression strings

Signed-off-by: Nils Bandener <nils.bandener@eliatra.com>

Author: nibix

server/src/main/java/org/opensearch/cluster/metadata/IndexNameExpressionResolver.java

@@ -1111,6 +1111,7 @@ private Set<String> innerResolve(Context context, List<String> expressions, Indi
                 String expression = expressions.get(i);
                 if (Strings.isEmpty(expression)) {
                     context.addResolutionError(indexNotFoundException(expression));
+                    continue;
                 }
                 validateAliasOrIndex(context, expression);
                 if (aliasOrIndexExists(context, options, metadata, expression)) {

server/src/test/java/org/opensearch/cluster/metadata/IndexNameExpressionResolverTests.java

@@ -2112,6 +2112,22 @@ public void testInvalidIndex() {
         assertEquals("Invalid index name [_foo], must not start with '_'.", iine.getMessage());
     }
 
+    public void testInvalidEmptyIndex() {
+        Metadata.Builder mdBuilder = Metadata.builder().put(indexBuilder("test"));
+        ClusterState state = ClusterState.builder(new ClusterName("_name")).metadata(mdBuilder).build();
+        IndexNameExpressionResolver.Context context = new IndexNameExpressionResolver.Context(
+            state,
+            IndicesOptions.lenientExpandOpen(),
+            false
+        );
+
+        IndexNotFoundException indexNotFoundException = expectThrows(
+            IndexNotFoundException.class,
+            () -> indexNameExpressionResolver.concreteIndexNames(context, "")
+        );
+        assertEquals("no such index []", indexNotFoundException.getMessage());
+    }
+
     public void testIgnoreThrottled() {
         Metadata.Builder mdBuilder = Metadata.builder()
             .put(