Use empty string as null value for FileEdit

[brisvag]

Fix for issue raised on zulip, where even when setting nullable=True, None would be treated as an actual path, as well as being used as a default value. This replaces it with the empty string.

cc @VolkerH

[codecov]

Codecov Report

Merging #384 (c5fcf7a) into main (0fa849a) will decrease coverage by 0.14%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##             main     #384      +/-   ##
==========================================
- Coverage   88.33%   88.18%   -0.15%     
==========================================
  Files          30       30              
  Lines        3610     3615       +5     
==========================================
- Hits         3189     3188       -1     
- Misses        421      427       +6     

Impacted Files	Coverage Δ
magicgui/widgets/_concrete.py	84.25% <71.42%> (-0.39%)	⬇️
magicgui/_type_wrapper.py	63.76% <100.00%> (-1.45%)	⬇️

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[tlambert03]

thanks! I'll deal with the typing tests later