Remove NPE from isClientError classification

NPE in the analytics path is a server bug (null schema field, missing
row), not bad user input. Removed from client error list. Will sync
this classification with RestPPLQueryAction updates in opensearch-project#5266.

Signed-off-by: Kai Huang <kaihuang@amazon.com>
Signed-off-by: Kai Huang <ahkcs@amazon.com>

Author: ahkcs

plugin/src/main/java/org/opensearch/sql/plugin/rest/RestUnifiedQueryAction.java

@@ -255,12 +255,12 @@ public void onFailure(Exception e) {
   }
 
   /**
-   * Classify whether the exception is a client error (bad query) or server error (engine bug).
-   * Matches the classification in {@link RestPPLQueryAction#isClientError}.
+   * Classify whether the exception is a client error (bad query) or server error (engine bug). Will
+   * be synced with RestPPLQueryAction#isClientError updates in
+   * https://github.com/opensearch-project/sql/pull/5266.
    */
   private static boolean isClientError(Exception e) {
-    return e instanceof NullPointerException
-        || e instanceof IllegalArgumentException
+    return e instanceof IllegalArgumentException
         || e instanceof IndexNotFoundException
         || e instanceof SemanticCheckException
         || e instanceof ExpressionEvaluationException