maildir_flags_to_tags: Avoid interpreting "no info" as "no flags set".

If a filename has no maildir info at all, (that is, it does not
contain the sequence ":2,"), we consider this distinct from a filename
with an empty maildir info, (the ":2," separator is present, but no
flags characters follow).

Specifically, we regard a missing info field as providing no
information, so tags will remain unchanged. On the other hand, an info
field that is present but has no flags set will cause various tags to
be cleared, (or in the case of "unread", added).

This fixes the "remove info" case of the maildir-sync tests in the
test suite.