refactor: consolidate empty-string env-var validators in Configuration

[vdusek]

Description

A few Configuration fields were already guarded against empty-string env vars with ad-hoc BeforeValidator lambdas. The Apify platform sometimes sets an env var to an empty string instead of leaving it unset (see #303, or this Discord thread), and for these fields '' cannot be parsed into the target type, so without the guard validation would raise and crash Actor.init().

This PR is a pure refactor: it consolidates those repeated lambdas into a single shared _default_if_empty(default=...) validator. There is no behavior change, and the empty-string handling is not extended to any new fields.

Changes

• Added a shared _default_if_empty(default=...) validator.
• Replaced the existing ad-hoc lambdas with it on the four fields that already handled '':
  • max_paid_dataset_items (→ None)
  • max_total_charge_usd (→ None)
  • timeout_at (→ None)
  • user_is_paying (→ False)
• Added a parametrized regression test covering those four fields.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.33%. Comparing base (0daca28) to head (6c90a1d).
⚠️ Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #965      +/-   ##
==========================================
+ Coverage   89.90%   90.33%   +0.42%     
==========================================
  Files          49       49              
  Lines        3091     3093       +2     
==========================================
+ Hits         2779     2794      +15     
+ Misses        312      299      -13     

Flag	Coverage Δ
e2e	36.27% <80.00%> (+0.36%)	⬆️
integration	57.48% <100.00%> (+0.60%)	⬆️
unit	78.95% <100.00%> (+0.20%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.