prevent utils.isPromise to crash when passing null

[Platane]

hello,

Due to typeof null === 'object the isPromise test is crashing when passing null

Here a demo: https://codesandbox.io/s/3vyq17zy35

This PR fix that

[codecov[bot]]

Codecov Report

Merging #61 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #61   +/-   ##
=======================================
  Coverage   91.22%   91.22%           
=======================================
  Files           6        6           
  Lines         114      114           
  Branches       34       34           
=======================================
  Hits          104      104           
  Misses          8        8           
  Partials        2        2

Impacted Files	Coverage Δ
src/utils.js	100% <100%> (ø)	:arrow_up:

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