fix: Always set _spotlight_script

The conditional early exit in `SpotlightMiddleware` may cause attribute access errors when trying to check if `_spotlight_script` is set or not. This patch sets it to `None` explicitly and always in the constructor to avoid/fix the issue.

Author: BYK

sentry_sdk/spotlight.py

@@ -86,6 +86,7 @@ class SpotlightMiddleware(MiddlewareMixin):  # type: ignore[misc]
         def __init__(self, get_response):
             # type: (Self, Callable[..., HttpResponse]) -> None
             super().__init__(get_response)
+            _spotlight_script = None
 
             import sentry_sdk.api