Correct nullability of function

[an0]

function should be nullable because aslMessageReceived clearly calls it with a null function.

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 necesarry)

• I have run the tests and they pass

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

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

Pull Request Description

...

[codecov-io]

Codecov Report

Merging #849 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #849   +/-   ##
=======================================
  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% <100%> (ø)	✅

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[bpoplauschi]

@an0 Thanks a lot for showing interest and contributing to CocoaLumberjack! Even tough it’s been a while, we would like to invite you to become a maintainer – no pressure to accept! You can pitch in with what seems comfortable: comment on open issues/PRs, triage, improve documentation, write your own PRs. See a broader discussion here #941. Let me know if you are interested.