Throw soft exception in case of empty signature while building Scan Query (#16502)

sql/src/main/java/org/apache/druid/sql/calcite/rel/DruidQuery.java

@@ -1602,7 +1602,8 @@ public class DruidQuery
 
     if (outputRowSignature.size() == 0) {
       // Should never do a scan query without any columns that we're interested in. This is probably a planner bug.
-      throw new ISE("Cannot convert to Scan query without any columns.");
+      this.plannerContext.setPlanningError("Cannot convert to Scan query without any columns.");
+      return null;
     }
 
     final Pair<DataSource, Filtration> dataSourceFiltrationPair = getFiltration(

sql/src/test/java/org/apache/druid/sql/calcite/CalciteWindowQueryTest.java

@@ -233,6 +233,32 @@ public class CalciteWindowQueryTest extends BaseCalciteQueryTest
     }
   }
 
+  @Test
+  public void testEmptyWindowInSubquery()
+  {
+    testBuilder()
+        .sql(
+            "select c from (\n"
+            + "  select channel, row_number() over () as c\n"
+            + "  from wikipedia\n"
+            + "  group by channel\n"
+            + ") LIMIT 5"
+        )
+        .queryContext(ImmutableMap.of(
+            PlannerContext.CTX_ENABLE_WINDOW_FNS, true,
+            QueryContexts.ENABLE_DEBUG, true,
+            QueryContexts.WINDOWING_STRICT_VALIDATION, false
+        ))
+        .expectedResults(ImmutableList.of(
+            new Object[]{1L},
+            new Object[]{2L},
+            new Object[]{3L},
+            new Object[]{4L},
+            new Object[]{5L}
+        ))
+        .run();
+  }
+
   @Test
   public void testWindow()
   {