fix(eslint-plugin): [req-await] crash on nonasync promise return

[bradzacher]

Fixes #1226

There wasn't a test case for a non-async function that returned a promise.

[typescript-eslint]

Thanks for the PR, @bradzacher!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #1228 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1228      +/-   ##
==========================================
- Coverage   94.23%   94.23%   -0.01%     
==========================================
  Files         126      126              
  Lines        5376     5375       -1     
  Branches     1509     1509              
==========================================
- Hits         5066     5065       -1     
  Misses        177      177              
  Partials      133      133

Impacted Files	Coverage Δ
packages/eslint-plugin/src/rules/require-await.ts	100% <100%> (ø)	⬆️