Fixed #35024 -- Fixed model instance creation crash on GeneratedField.output_field with backend converters.

Regression in d9de74141e.

This is a long standing issue, however it caused a crash of
GeneratedFields for all output fields that have backend-specific
converters when the RETURNING clause is not supported
(MySQL and SQLite < 3.35).
That's why severity was exacerbated.

django/db/models/sql/compiler.py

@@ -1819,6 +1819,7 @@ class SQLInsertCompiler(SQLCompiler):
         )
         opts = self.query.get_meta()
         self.returning_fields = returning_fields
+        cols = []
         with self.connection.cursor() as cursor:
             for sql, params in self.as_sql():
                 cursor.execute(sql, params)
@@ -1829,6 +1830,7 @@ class SQLInsertCompiler(SQLCompiler):
                 and len(self.query.objs) > 1
             ):
                 rows = self.connection.ops.fetch_returned_insert_rows(cursor)
+                cols = [field.get_col(opts.db_table) for field in self.returning_fields]
             elif self.connection.features.can_return_columns_from_insert:
                 assert len(self.query.objs) == 1
                 rows = [
@@ -1837,7 +1839,9 @@ class SQLInsertCompiler(SQLCompiler):
                         self.returning_params,
                     )
                 ]
+                cols = [field.get_col(opts.db_table) for field in self.returning_fields]
             else:
+                cols = [opts.pk.get_col(opts.db_table)]
                 rows = [
                     (
                         self.connection.ops.last_insert_id(
@@ -1847,7 +1851,6 @@ class SQLInsertCompiler(SQLCompiler):
                         ),
                     )
                 ]
-        cols = [field.get_col(opts.db_table) for field in self.returning_fields]
         converters = self.get_converters(cols)
         if converters:
             rows = list(self.apply_converters(rows, converters))