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

[vdusek]

Description

• Basically be more defensive and robust (see e.g. fix: Allow empty timeout_at env variable #303, or https://discord.com/channels/801163717915574323/1020281045205123102/threads/1294774068528156817), the Apify platform can set ACTOR_STORAGES_JSON and APIFY_CHARGED_ACTOR_EVENT_COUNTS to an empty string instead of leaving them unset. Both validators passed the value straight to json.loads(''), which raises JSONDecodeError and crashes Actor.init().
• Treat '' as absent in _load_storage_keys and in the charged_event_counts validator, mirroring the existing _parse_actor_pricing_info behavior.
• Add regression tests for both env vars.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.88%. Comparing base (2cc5602) to head (98c59cb).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #965      +/-   ##
==========================================
- Coverage   89.91%   89.88%   -0.04%     
==========================================
  Files          49       49              
  Lines        3085     3085              
==========================================
- Hits         2774     2773       -1     
- Misses        311      312       +1     

Flag	Coverage Δ
e2e	35.98% <0.00%> (ø)
integration	56.95% <100.00%> (-0.04%)	⬇️
unit	78.70% <100.00%> (ø)

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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...