Decrypt: check for null values to avoid exception on Encoding.UTF8.GetString

[romangobrey]

Fix: provider might return null (for decryptedRawBytes variable) when decrypting data which leads to an exception here

var decryptedData = Encoding.UTF8.GetString(decryptedRawBytes).Trim('\0');

[codecov-commenter]

Codecov Report

Merging #56 (fc504f9) into main (ea6836b) will not change coverage. The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main      #56   +/-   ##
=======================================
  Coverage   94.44%   94.44%           
=======================================
  Files           6        6           
  Lines         144      144           
  Branches       30       30           
=======================================
  Hits          136      136           
  Misses          3        3           
  Partials        5        5           

Impacted Files	Coverage Δ
...ore.DataEncryption/Internal/EncryptionConverter.cs	93.93% <100.00%> (ø)