Do no fail if the source attribute is undefined in an unused named export

[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 #3210

Description

This is a quick fix by changing how Rollup checks for named re-exports, but it is recommended that rollup-plugin-dts improves the ESTree compliance of their generated ASTs.

[codecov]

Codecov Report

Merging #3211 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3211   +/-   ##
=======================================
  Coverage   90.64%   90.64%           
=======================================
  Files         167      167           
  Lines        5909     5909           
  Branches     1792     1792           
=======================================
  Hits         5356     5356           
  Misses        336      336           
  Partials      217      217

Impacted Files	Coverage Δ
src/Module.ts	97.31% <100%> (ø)	⬆️

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