fix(search): added if-null check for playlist video count

[codedbycurtis]

This reverts to the previous functionality of checking for null and returning 0 when parsing a playlist's video count from the search page. Some auto-generated playlists currently return Mix, rather than an actual video count, so in future, perhaps it would make sense to add some sort of indicator to SearchPlaylist to denote this, or even return -1 instead.

Closes #315.

[Hexer10]

Thank you!