Update comment

evadb/executor/executor_utils.py

@@ -38,6 +38,7 @@ class ExecutorError(Exception):
 def apply_project(
     batch: Batch, project_list: List[AbstractExpression], catalog: "CatalogManager"
 ):
+    # batch is None for `SELECT expr;`, where we don't have input table relationship.
     if batch is None or (not batch.empty() and project_list):
         batches = [expr.evaluate(batch) for expr in project_list]
         batch = Batch.merge_column_wise(batches)