fix static type check

Signed-off-by: Sai Shree Pradhan <saishree.pradhan@databricks.com>

Author: saishreeeee

src/databricks/sql/common/feature_flag.py

@@ -94,6 +94,8 @@ def is_feature_enabled(self, name: str, default_value: bool) -> bool:
                 # We don't check for an in-flight refresh; the executor queues the task, which is safe.
                 self._executor.submit(self._refresh_flags)
 
+            assert self._flags is not None
+
             # Now, return the value from the populated cache.
             flag_value = self._flags.get(name)
             if flag_value is None: