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

Merging #1945 (e4511c5) into develop (067aca2) will decrease coverage by 0.0%.
The diff coverage is 0.0%.

Additional details and impacted files

@@             Coverage Diff             @@
##             develop   #1945     +/-   ##
===========================================
- Coverage       56.8%   56.8%   -0.0%     
- Complexity      4101    4102      +1     
===========================================
  Files            456     456             
  Lines          17621   17623      +2     
===========================================
  Hits           10011   10011             
- Misses          7610    7612      +2     

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

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