fix: Fix BeforeValidator treating 0 as falsy in configuration fields

`lambda val: val or None` coerces `0` and `Decimal(0)` to `None`,
making it impossible to set `max_paid_dataset_items=0` or
`max_total_charge_usd=0`. Use explicit empty-string check instead,
consistent with the existing `timeout_at` validator.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: vdusek

src/apify/_configuration.py

@@ -295,7 +295,7 @@ class Configuration(CrawleeConfiguration):
             alias='actor_max_paid_dataset_items',
             description='For paid-per-result Actors, the user-set limit on returned results. Do not exceed this limit',
         ),
-        BeforeValidator(lambda val: val or None),
+        BeforeValidator(lambda val: val if val != '' else None),
     ] = None
 
     max_total_charge_usd: Annotated[
@@ -304,7 +304,7 @@ class Configuration(CrawleeConfiguration):
             alias='actor_max_total_charge_usd',
             description='For pay-per-event Actors, the user-set limit on total charges. Do not exceed this limit',
         ),
-        BeforeValidator(lambda val: val or None),
+        BeforeValidator(lambda val: val if val != '' else None),
     ] = None
 
     test_pay_per_event: Annotated[