[API Pull] Ensure empty GLA attributes are returned as an object instead of an empty array in the responses.

[jorgemd24]

Changes proposed in this Pull Request:

Part of #2146

This PR ensures that empty GLA attributes are returned as an empty object instead of an empty array, maintaining consistency in the responses.

Screenshots:

Detailed test instructions:

1. Remove any attribute mapping set.
2. Make a request to GET wp-json/wc/v3/products/PRODUCT_ID?gla_syncable=1
3. Check that gla_attributes is returned as an empty object instead of an empty array.

Additional details:

Changelog entry

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.7%. Comparing base (d47c264) to head (0642055).
Report is 1 commits behind head on feature/google-api-project.

Additional details and impacted files

@@                      Coverage Diff                       @@
##             feature/google-api-project   #2455     +/-   ##
==============================================================
+ Coverage                          64.4%   64.7%   +0.3%     
  Complexity                         4551    4551             
==============================================================
  Files                               795     473    -322     
  Lines                             22796   17753   -5043     
  Branches                           1219       0   -1219     
==============================================================
- Hits                              14688   11484   -3204     
+ Misses                             7941    6269   -1672     
+ Partials                            167       0    -167     

Flag	Coverage Δ
js-unit-tests	?
php-unit-tests	64.7% <100.0%> (ø)

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

Files	Coverage Δ
src/Integration/WPCOMProxy.php	70.2% <100.0%> (ø)

... and 322 files with indirect coverage changes

[puntope]

LGTM ✅ I can see the empty array is now an empty object