fix(lib/cli): relative paths, empty slides, and tests

[jeertmans]

This fixes two issues:

1. Empty slides are now reported as error, to prevent indexing error;
2. Changing the folder path will now produce an absolute path to slides, which was not the case before and would lead to a "file does not exist error".

A few tests were also added to cover those

[codecov]

Codecov Report

Patch coverage: 75.67% and project coverage change: +18.31 🎉

Comparison is base (2b6240c) 39.97% compared to head (fc99f44) 58.29%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #223       +/-   ##
===========================================
+ Coverage   39.97%   58.29%   +18.31%     
===========================================
  Files          14       14               
  Lines        1691     1712       +21     
===========================================
+ Hits          676      998      +322     
+ Misses       1015      714      -301     

Impacted Files	Coverage Δ
manim_slides/wizard.py	37.39% <50.00%> (+37.39%)	⬆️
manim_slides/config.py	74.71% <82.60%> (+14.32%)	⬆️
manim_slides/present.py	56.37% <83.33%> (+29.38%)	⬆️

... and 2 files with indirect coverage changes

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