Refactor to get the first element independently of its key

[puntope]

Changes proposed in this Pull Request:

Related to #1943

Even tho this is likely not happening because Wp_Terms are ordered starting from 0. I added this fix to be sure we return the first element of the array independently of the key.

That way we can be sure we can prevent the warning of Warning: Undefined array key 0

Detailed test instructions:

1. Create a new attribute mapping rule with a taxonomy Color
2. See the attribute mapping rule working as normally.

ℹ️ For not waiting the 30 minutes in order to sync the rules you can sync a specific product via connection test.

Additional details:

Changelog entry

Fix - Prevent potential warning when accessing undefined index.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 56.8%. Comparing base (a07bd5f) to head (e4511c5).
Report is 2584 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/Product/WCProductAdapter.php	0.0%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             develop   #1945     +/-   ##
===========================================
- Coverage       56.8%   56.8%   -0.0%     
- Complexity      4101    4102      +1     
===========================================
  Files            456     456             
  Lines          17620   17623      +3     
===========================================
+ Hits           10010   10011      +1     
- Misses          7610    7612      +2     

Flag	Coverage Δ
php-unit-tests	56.8% <0.0%> (-<0.1%)	⬇️

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

Files with missing lines	Coverage Δ
src/Product/WCProductAdapter.php	91.5% <0.0%> (-0.4%)	⬇️

... and 1 file with indirect coverage changes