fix: Allow empty timeout_at env variable (#303)

- closes apify/crawlee-python#596

Author: janbuchar

src/apify/_configuration.py

@@ -251,6 +251,7 @@ class Configuration(CrawleeConfiguration):
             ),
             description='Date when the Actor will time out',
         ),
+        BeforeValidator(lambda val: val if val != '' else None),  # We should accept empty environment variables as well
     ] = None
 
     standby_port: Annotated[