Fix emitted FIRRTL for dynamic index of size 0 Vec (backport #4275)

[mergify]

#4268 introduced a subtle bug for when dynamically indexing Vecs of size 0. This is currently deprecated but obviously we need to emit legal FIRRTL until it becomes an error.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
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• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Bugfix (but no release notes because this bug isn't released)

Desired Merge Strategy

• Squash

Release Notes

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
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• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
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This is an automatic backport of pull request #4275 done by [Mergify](https://mergify.com).