fix: proto UnmarshalValue on handling string padding

[muktihari]

Strings may contains padding bytes. Previously, I thought that the padding only consist of zero bytes, but it turned out the padding bytes can be vary for example workout_step's notes (array: false):

Since string in utf8 is terminated by zero, we cut the value up to zero. Now we will omit invalid utf8 character as well, since it does not comply with FIT's string spec.

It was first discovered when handling files from this thread:
https://forums.garmin.com/developer/fit-sdk/f/discussion/383991/my-fit-file-that-i-combined-won-t-upload-to-garmin-connect#pifragment-1304=1

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e2c16ec) to head (6ba6b98).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #436   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files           42        42           
  Lines         3658      3659    +1     
=========================================
+ Hits          3658      3659    +1     

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