Minor tweaks (#10987)

### What problem does this PR solve?

1. Rename identifier name
2. Fix some return statement
3. Fix some typos

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

api/db/db_models.py

@@ -369,6 +369,7 @@ class RetryingPooledPostgresqlDatabase(PooledPostgresqlDatabase):
                     time.sleep(self.retry_delay * (2 ** attempt))
                 else:
                     raise
+        return None
 
 
 class PooledDatabase(Enum):