Do not fail when returning null or undefined from an async options hook

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:
Resolves #4038

Description

Due to a missing await, async options hooks with false results were not handled correctly.

[github-actions]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#gh-4038-async-options-null

or load it into the REPL:
https://rollupjs.org/repl/?circleci=14933

[codecov]

Codecov Report

Merging #4039 (7bb6839) into master (5a22c8a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4039   +/-   ##
=======================================
  Coverage   97.47%   97.47%           
=======================================
  Files         192      192           
  Lines        6785     6785           
  Branches     1994     1994           
=======================================
  Hits         6614     6614           
  Misses         84       84           
  Partials       87       87           

Impacted Files	Coverage Δ
src/rollup/rollup.ts	100.00% <100.00%> (ø)

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