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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