TopWiring: filter out unnamed declarations when building source lists

[davidbiancolin]

TopWiring croaks when building up a potential source list because it tries to generate a ComponentName for declarations whose name is the empty string. Since unnamed declarations cannot be targeted anyways (i think?) simply filter them out. This will give a slightly better error message when trying to wire an unsupported declaration.

Subsumes #2372.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you update the FIRRTL spec to include every new feature/behavior?
• 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 state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• bug fix

API Impact

No change.

Backend Code Generation Impact

No change.

Desired Merge Strategy

• Squash

Release Notes

TopWiring: filter out unnamed declarations when building source lists

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (1.2.x, 1.3.0, 1.4.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?

[ekiwi]

LGTM! Thanks for adding the test!