fix: treat empty-string env vars as absent in Configuration

[vdusek]

Description

The Apify platform sometimes sets env vars to an empty string instead of leaving them unset (see #303, or this Discord thread). For fields whose type can't parse '', validation raised and crashed Actor.init(). Treat '' as absent / fall back to the field default, mirroring the existing _parse_actor_pricing_info behavior.

• JSON fields (ACTOR_STORAGES_JSON, APIFY_CHARGED_ACTOR_EVENT_COUNTS): json.loads('') raised JSONDecodeError. Now treated as absent in _load_storage_keys and the charged_event_counts validator.
• Typed fields (datetime / number / bool / timedelta): '' failed pydantic validation. Added a shared _default_if_empty validator and applied it to started_at, dedicated_cpus, proxy_port, web_server_port, metamorph_after_sleep, is_at_home, test_pay_per_event — and consolidated the existing ad-hoc lambdas (timeout_at, max_paid_dataset_items, max_total_charge_usd, user_is_paying).
• Regression tests for all affected env vars.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.91%. Comparing base (0daca28) to head (5cc2a2a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #965      +/-   ##
==========================================
+ Coverage   89.90%   89.91%   +0.01%     
==========================================
  Files          49       49              
  Lines        3091     3095       +4     
==========================================
+ Hits         2779     2783       +4     
  Misses        312      312              

Flag	Coverage Δ
e2e	35.96% <66.66%> (+0.05%)	⬆️
integration	56.93% <100.00%> (+0.05%)	⬆️
unit	78.77% <100.00%> (+0.02%)	⬆️

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[Pijukatel]

Isn't this actually less defensive?

Is there any actual use case for having 'ACTOR_STORAGES_JSON' be set as an empty string? The env variable clearly states it should be a json. So either have json there, or don't set the variable.

Code should not try to guess the intention from an ambiguous action. And here we are guessing that an empty env variable probably means the user did not want to set it up in the first place...