Check sqlite database file exists before porting. (#14692)

To avoid creating an empty SQLite file if the given path
is incorrect.

changelog.d/14692.misc

+Check that the SQLite database file exists before porting to PostgreSQL.
\ No newline at end of file

synapse/_scripts/synapse_port_db.py

@@ -1307,7 +1307,7 @@ def main() -> None:
     sqlite_config = {
         "name": "sqlite3",
         "args": {
-            "database": args.sqlite_database,
+            "database": "file:{}?mode=rw".format(args.sqlite_database),
             "cp_min": 1,
             "cp_max": 1,
             "check_same_thread": False,