Add test for missing required option validation path

Signed-off-by: Eric Wei <mengwei.eric@gmail.com>

Author: mengweieric

opensearch/src/test/java/org/opensearch/sql/opensearch/storage/VectorSearchTableFunctionImplementationTest.java

@@ -138,6 +138,15 @@ void testDuplicateOptionKeyThrows() {
     assertTrue(ex.getMessage().contains("Duplicate option key"));
   }
 
+  @Test
+  void testNoRequiredOptionThrows() {
+    VectorSearchTableFunctionImplementation impl =
+        createImplWithArgs("my-index", "embedding", "[1.0, 2.0]", "");
+    ExpressionEvaluationException ex =
+        assertThrows(ExpressionEvaluationException.class, () -> impl.applyArguments());
+    assertTrue(ex.getMessage().contains("Missing required option"));
+  }
+
   @Test
   void testEmptyVectorThrows() {
     VectorSearchTableFunctionImplementation impl =