Fixed nullability of DDExtractFileNameWithoutExtension() function.

[jcbertin]

New Pull Request Checklist

• I have read and understood the CONTRIBUTING guide

• I have read the Documentation

• I have searched for a similar pull request in the project and found none

• I have updated this branch with the latest master to avoid conflicts (via merge from master or rebase)

• I have added the required tests to prove the fix/feature I am adding

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This merge request fixes / reffers to the following issues: ...

Pull Request Description

I fixed the signature of DDExtractFileNameWithoutExtension() function since it can returns null values if filePath parameter is null or if -[NSString initWithBytes:] fails.
I also renamed LOG_MAX_QUEUE_SIZE to DDLOG_MAX_QUEUE_SIZE and make it optional. It will not break existing code since this macro is private (see Discussion #839).

[codecov-io]

Codecov Report

Merging #845 into master will not change coverage.
The diff coverage is 80%.

@@           Coverage Diff           @@
##           master     #845   +/-   ##
=======================================
  Coverage   23.07%   23.07%           
=======================================
  Files          16       16           
  Lines        3913     3913           
  Branches      279      279           
=======================================
  Hits          903      903           
  Misses       3008     3008           
  Partials        2        2

Impacted Files	Coverage Δ
Classes/DDLog.h	88.23% <ø> (ø)	✅
Classes/DDLog.m	57.18% <80%> (ø)	✅

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