Do not append always empty restore_key

[szepeviktor]

Description

It was just initialized: restore_key=()

PR checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #216 (e1277c4) into v2 (ef0ac57) will decrease coverage by 1.61%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##               v2     #216      +/-   ##
==========================================
- Coverage   88.70%   87.09%   -1.62%     
==========================================
  Files           5        5              
  Lines         124      124              
==========================================
- Hits          110      108       -2     
- Misses         14       16       +2     

Impacted Files	Coverage Δ
bin/cache_key.sh	76.92% <100.00%> (ø)
bin/composer_paths.sh	85.00% <0.00%> (-5.00%)	⬇️

[ramsey]

Thanks. This is leftover from when I was creating multiple restore keys, which I might revisit in the future, but it was causing too many cache collisions.