refactor: change error type for "no statement"

Amends apache#11394 (sorry, I should have reviewed that).

While reporting "not implemented" for "multiple statements" seems
reasonable, I think the user should get a plan error (which roughly
translates to "invalid argument") if they don't provide any statement. I
don't see any reasonable way to support "no statement" ever, hence "not
implemented" seems like a wrong promise.

datafusion/core/src/execution/session_state.rs

@@ -554,7 +554,7 @@ impl SessionState {
             );
         }
         let statement = statements.pop_front().ok_or_else(|| {
-            DataFusionError::NotImplemented(
+            DataFusionError::Plan(
                 "No SQL statements were provided in the query string".to_string(),
             )
         })?;

datafusion/core/tests/sql/sql_api.rs

@@ -124,7 +124,7 @@ async fn empty_statement_returns_error() {
     let plan_res = state.create_logical_plan("").await;
     assert_eq!(
         plan_res.unwrap_err().strip_backtrace(),
-        "This feature is not implemented: No SQL statements were provided in the query string"
+        "Error during planning: No SQL statements were provided in the query string"
     );
 }