Fix nullability violations

[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

• I have updated the documentation (if necessary)

• I have run the tests and they pass

• I have run the lint and it passes (pod lib lint)

Pull Request Description

After enabling nullability violations checks, I found an issue where _currentLogFileInfo could be used before being initialized. This PR fixes that.

[codecov]

Codecov Report

Merging #1287 (b248832) into master (cff9d3e) will decrease coverage by 0.00%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master    #1287      +/-   ##
==========================================
- Coverage   48.26%   48.26%   -0.01%     
==========================================
  Files          32       32              
  Lines        5200     5203       +3     
  Branches      488      489       +1     
==========================================
+ Hits         2510     2511       +1     
- Misses       2501     2504       +3     
+ Partials      189      188       -1     

Impacted Files	Coverage Δ
Sources/CocoaLumberjack/DDFileLogger.m	52.56% <25.00%> (-0.06%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cff9d3e...b248832. Read the comment docs.

[ffried]

Looks good. Just have one small question (see inline).