[Error Enhancement] Fix NPE when rex sits inside appendcol subsearch for Analytic Engine (#5574)

Signed-off-by: Jialiang Liang <jiallian@amazon.com>

Author: RyanL1997

core/src/main/java/org/opensearch/sql/ast/tree/Rex.java

@@ -79,7 +79,7 @@ public Rex attach(UnresolvedPlan child) {
 
   @Override
   public List<UnresolvedPlan> getChild() {
-    return ImmutableList.of(child);
+    return this.child == null ? ImmutableList.of() : ImmutableList.of(this.child);
   }
 
   @Override

integ-test/src/test/java/org/opensearch/sql/calcite/remote/CalcitePPLAppendcolIT.java

@@ -81,4 +81,18 @@ public void testAppendColOverride() throws IOException {
         rows("F", "DE", 101, null),
         rows("F", "FL", 310, null));
   }
+
+  /** Verifies that rex can be used as the first command of an appendcol subsearch. */
+  @Test
+  public void testAppendColWithRexInSubsearch() throws IOException {
+    JSONObject actual =
+        executeQuery(
+            String.format(
+                "source=%s | stats count() as cnt by gender"
+                    + " | appendcol [ rex field=email '^(?<user>[^@]+)@.*' | fields user ]"
+                    + " | head 2",
+                TEST_INDEX_ACCOUNT));
+    verifySchema(
+        actual, schema("gender", "string"), schema("cnt", "bigint"), schema("user", "string"));
+  }
 }

ppl/src/test/java/org/opensearch/sql/ppl/calcite/CalcitePPLRexTest.java

@@ -306,4 +306,14 @@ public void testRexWithMaxMatchAndOffsetField() {
             + "FROM `scott`.`EMP`";
     verifyPPLToSparkSQL(root, expectedSparkSql);
   }
+
+  /** Verifies that rex plans correctly when it appears as the first command of a subsearch. */
+  @Test
+  public void testRexInsideSubsearch() {
+    String ppl =
+        "source=EMP | stats count() as base_c by JOB"
+            + " | appendcol [ rex field=ENAME '^(?<first>[A-Z])' | fields ENAME, first ]";
+    RelNode root = getRelNode(ppl);
+    org.junit.Assert.assertNotNull(root);
+  }
 }