Check whether provided base64 encoded data in KubeConfig is valid

[stefankonig]

Hi again, thanks for the quick response on my last PR.

I came across another minor thing, while testing i inadvertently used an invalid base64 encoded certificate in my kube config. This did not result in a clear error message, as the file_put_contents function successfully wrote the bool false as contents of the certificate.pem file. Later down the line this resulted in an less obvious Curl error 58.

Therefore i added a check to make sure it at least was able to decoding something from the base64 contents.

[codecov]

Codecov Report

Merging #183 (9edef5f) into master (9f72121) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #183   +/-   ##
=========================================
  Coverage     95.19%   95.20%           
- Complexity      693      694    +1     
=========================================
  Files            70       70           
  Lines          1685     1688    +3     
=========================================
+ Hits           1604     1607    +3     
  Misses           81       81           

Impacted Files	Coverage Δ
src/Traits/Cluster/LoadsFromKubeConfig.php	93.75% <100.00%> (+0.24%)	⬆️

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