Properly handle default exporting "undefined"

[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 #3557

Description

This will make sure that the default export simplification logic does not kick in when undefined is exported, leading to a crash.

[rollup-bot]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#gh-3557-undefined-default-export

or load it into the REPL:
https://rollupjs.org/repl/?circleci=11061

[codecov]

Codecov Report

Merging #3558 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3558   +/-   ##
=======================================
  Coverage   96.29%   96.29%           
=======================================
  Files         176      176           
  Lines        6048     6049    +1     
  Branches     1783     1784    +1     
=======================================
+ Hits         5824     5825    +1     
  Misses        112      112           
  Partials      112      112           

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

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