fix: correctly strip/restore initial punctuation (#3336)

* refactor(punctuation): remove orphan code for handling lone punctuation

The case of lone punctuation is already handled at the top of restore(). The
removed if statement would never be called and would in fact raise an
AttributeError because the _punc_index named tuple doesn't have the attribute
`mark`.

* refactor(punctuation): remove unused argument

* fix(punctuation): correctly handle initial punctuation

Stripping and restoring initial punctuation didn't work correctly because the
string-splitting caused an additional empty string to be inserted in the text
list (because `".A".split(".")` => `["", "A"]`). Now, an initial empty string is
skipped and relevant test cases are added.

Fixes #3333