types: correct type predicate

[rzzf]

demo

const foo = () => {
  const bar: number | undefined = 1
  if (isNullish(bar)) {
    return
  }
 
  // Before: The type of bar is never.
  // After: The type of bar is number.
}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.61%. Comparing base (c10d263) to head (044b200).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev      #43   +/-   ##
=======================================
  Coverage   99.61%   99.61%           
=======================================
  Files         132      132           
  Lines         773      773           
  Branches      156      156           
=======================================
  Hits          770      770           
  Misses          3        3           

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