mpd module: added support for custom artist name if artist is empty

[koraynilay]

This is my first time contributing to a project through github, so I hope I did everything correctly. Feel free to critique my code (obviously) since it's my first time using c++ properly (I only used C).

The code works tested (should work also on the master branch, but #2075 ) with the latest tarball (sha256sum: 4d22c977969a561f561fdc7a609073854d8fea8a9eec6941e12a80457edcb63a).

To use it you add

;it doesn't matter the order of <unkartist-song>, it just needs to be there
format-online = <unkartist-song> <label-song>

label-song = %cartist% %title%
unkartist-song = whatever

[codecov]

Codecov Report

Merging #2076 into master will increase coverage by 0.21%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master   #2076      +/-   ##
=========================================
+ Coverage    9.39%   9.61%   +0.21%     
=========================================
  Files         181     182       +1     
  Lines       10873   11603     +730     
=========================================
+ Hits         1022    1116      +94     
- Misses       9851   10487     +636     

Flag	Coverage Δ
#unittests	9.61% <0.00%> (+0.21%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
include/modules/mpd.hpp	0.00% <ø> (ø)
src/modules/mpd.cpp	0.00% <0.00%> (ø)
include/components/logger.hpp	38.88% <0.00%> (-2.29%)	⬇️
src/ipc.cpp	0.00% <0.00%> (ø)
src/modules/menu.cpp	0.00% <0.00%> (ø)
include/x11/winspec.hpp	0.00% <0.00%> (ø)
include/adapters/net.hpp	0.00% <0.00%> (ø)
include/events/signal.hpp	0.00% <0.00%> (ø)
include/utils/command.hpp	66.66% <0.00%> (ø)
src/modules/backlight.cpp	0.00% <0.00%> (ø)
... and 20 more

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