Added unit tests for transformReadyToAbsolute()

[vershwal]

ref https://linear.app/ghost/issue/PRO-1513 https://linear.app/ghost/issue/PRO-1517

This PR adds the following edge case tests for transformReadyToAbsolute()

✅ Empty string → empty string
✅ null → null
✅ undefined -> empty string (normalized by default parameter)
✅ Malformed URL in content → graceful fallback
✅ Only media CDN set (files CDN = null):

• /content/media/ → uses CDN
• /content/files/ → uses site URL
• /content/images/ → uses site URL

✅ Empty CDN config (urls.media = null) → all use site URL

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.62%. Comparing base (969c43b) to head (cc7d18d).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #712      +/-   ##
==========================================
- Coverage   86.46%   84.62%   -1.85%     
==========================================
  Files          75       33      -42     
  Lines        5335     3518    -1817     
  Branches      706      394     -312     
==========================================
- Hits         4613     2977    -1636     
+ Misses        715      536     -179     
+ Partials        7        5       -2     

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[jloh]

Nice, LGTM!