fix function with no params and callbutton

[tlambert03]

This fixes a bug when creating a vertically oriented magicgui with call_button=True, when there are no parameters in the function

closes #146

[codecov]

Codecov Report

Merging #149 (a9a9333) into master (f2955c3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #149   +/-   ##
=======================================
  Coverage   91.24%   91.25%           
=======================================
  Files          25       25           
  Lines        2502     2503    +1     
=======================================
+ Hits         2283     2284    +1     
  Misses        219      219           

Impacted Files	Coverage Δ
magicgui/widgets/_bases/container_widget.py	94.93% <100.00%> (+0.03%)	⬆️

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