Empty tags are serialised incorrectly, causes UI issue in Buku

[samhh]

Here's how we're currently serialising our array of tags back to the format that Buku/SQLite want:

delimiter + bookmark.tags.join(delimiter) + delimiter

This is correct when bookmark.tags is non-empty (e.g. ,apple,, ,apple,banana,, etc), but incorrect when it's empty. What we want is ,, and what we actually produce is ,,.

Whilst this doesn't outright crash Buku or anything, it does result in the bookmarks displaying incorrectly - displaying empty tags where it would otherwise hide them i.e.:

What we should see in Buku:

1. title [123]
   > url

What we actually see:

1. title [123]
   > url
   #

[samhh]

Trivially fixed in purescript branch, but not yet addressed in TypeScript/master.