Fix RangeWidget with implicit optional type

[tlambert03]

This fixes failed initialization of a widget with an implied optional value, that now happens in 0.2.9:

def func(x: int = None): ...

still not sure this is the best solution, but it at least brings us back to the 0.2.8 behavior where that would make a widget with a value of 0.

[codecov]

Codecov Report

Merging #257 (f155097) into master (4b71fc1) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #257   +/-   ##
=======================================
  Coverage   90.68%   90.68%           
=======================================
  Files          28       28           
  Lines        3103     3103           
=======================================
  Hits         2814     2814           
  Misses        289      289           

Impacted Files	Coverage Δ
magicgui/widgets/_bases/ranged_widget.py	85.56% <100.00%> (ø)

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