Do not create invalid code if a function argument is an empty object pattern

[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 #3997

Description

When a function argument is an empty object pattern, Rollup would see that no variables are declared and falsely assume that it is not necessary. This fix adds special handling for that case (note that when even a single, even unused, variable was declared in the pattern, it was retained correctly).

[rollup-bot]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#gh-3997_empty_object_destructuring

or load it into the REPL:
https://rollupjs.org/repl/?circleci=14534

[codecov]

Codecov Report

Merging #3998 (25dc45e) into master (770da18) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3998   +/-   ##
=======================================
  Coverage   97.22%   97.22%           
=======================================
  Files         191      191           
  Lines        6743     6745    +2     
  Branches     1969     1970    +1     
=======================================
+ Hits         6556     6558    +2     
  Misses         99       99           
  Partials       88       88           

Impacted Files	Coverage Δ
src/ast/scopes/ParameterScope.ts	100.00% <100.00%> (ø)

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