Fix "Cannot use empty key for Dictionary" error

This pull request resolves a compatibility issue with [fugitive](https://github.com/tpope/vim-fugitive) plugin. When using "Gdiff" command on some file, fugitive attempts to open the current and the index version of the file.  For example, if the file is

    /home/taketwo/path/to/the/project/README.md

then fugitive will use the following path to open the index version:

    fugitive:///home/taketwo/path/to/the/project/.git//0/README.md

Opening this file leads to a cascade of errors like this:

    Error detected while processing function
    airline#extensions#tabline#get..<SNR>162_get_buffers..<SNR>162_get_visible_buffers..airline#extensions#tabline#get_buffer_name..airline#extensions#tabline#unique_tail_improved#format:
    line   20:
    E713: Cannot use empty key for Dictionary

The problem is that when a filename like this is being tokenized, there appear empty "" tokens, and an error occurs when using them as keys in `path_tokens[token_index]` dictionary. This fix simply skips empty tokens.