barplot/collapse: ignore trailing semicolon in annotations

[jairideout]

Bug Description
Some reference databases (e.g. PR2) include a trailing semicolon in their taxonomic annotations. barplot parses this trailing semicolon as an extra "empty" taxonomic level.

Expected Behavior
It'd be great to ignore trailing semicolons to avoid creating an empty level in the plots.

Comments
NOTE: I haven't tested what collapse does with trailing semicolons. Whoever is tackling this issue, please also test collapse -- we'd want to avoid collapsing at this trailing "empty" level (raising an error in this case sounds reasonable).

References
Came up on the forum here.