Fix/prefix ids sameline comments

[john-neptune]

#1941

Fix prefixing classes on same line as a comment

Before, this worked:

<style>
.cls-1 {}
<!-- foo -->
.cls-2 {}
</style>

but this failed:

<style>
.cls-1 {} <!-- foo -->
.cls-2 {}
</style>

.cls-1 would be prefixed, but .cls-2 and subsquent classes would not.

The root cause is an empty return that was missed during a refactor to
switch from forEach() with a lambda to a for () loop.

Add a new test to cover this case.

[SethFalco]

LGTM! Thanks for reporting the issue and submitting a patch. This will be in the next release of SVGO.
Also, sorry for breaking your implementation during the rebase of #1736. ^-^'