Fix UnboundLocalError when checking alerts for query (#7346)

This fixes the following exception:

```
UnboundLocalError: local variable 'value_is_number' referenced before assignment
  File "rq/worker.py", line 1431, in perform_job
    rv = job.perform()
  File "rq/job.py", line 1280, in perform
    self._result = self._execute()
  File "rq/job.py", line 1317, in _execute
    result = self.func(*self.args, **self.kwargs)
  File "redash/tasks/alerts.py", line 36, in check_alerts_for_query
    new_state = alert.evaluate()
  File "redash/models/__init__.py", line 1002, in evaluate
    new_state = next_state(op, value, threshold)
  File "redash/models/__init__.py", line 928, in next_state
    elif not value_is_number and op not in [OPERATORS.get("!="), OPERATORS.get("=="), OPERATORS.get("equals")]:
```

Author: bluetech

redash/models/__init__.py

@@ -908,6 +908,7 @@ def next_state(op, value, threshold):
         # boolean value is Python specific and most likely will be confusing to
         # users.
         value = str(value).lower()
+        value_is_number = False
     else:
         try:
             value = float(value)