Add undefined and null as allowed modifier types

[LavaToaster]

Examples like: https://react-day-picker.js.org/examples/selected-range-enter/ show that modifiers are in fact supported as undefined or nullable. They do not work in typescript without some fair amount of work to ensure the value isn't null, when in fact the library does handle those types.

[codecov]

Codecov Report

Merging #775 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #775   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files          15       15           
  Lines         646      646           
  Branches      142      142           
=======================================
  Hits          645      645           
  Misses          1        1

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[gpbl]

Thanks! We are planning a release in the next days, so stay tuned.

[LavaToaster]

Ah excellent news :)