empty uri's crashes plugin; nil string in _is_root_path

[TamaMcGlinn]

When opening a markdown file with invalid image uri's, like this:

![something]()

I'm getting this error message:

Error detected while processing function dirvish#open[14]..<SNR>356_open_selected[22]..BufWinEnter Autocommands for "*":                                                                                       
Error executing lua callback: ...tama/.vim/plugged/hologram.nvim/lua/hologram/init.lua:120: bad argument #1 to 'sub' (string expected, got nil)                                                             
stack traceback:                                                                                                                                                                                               
        [C]: in function 'sub'                                                                                                                                                                                 
        ...tama/.vim/plugged/hologram.nvim/lua/hologram/init.lua:120: in function '_is_root_path'                                                                                                           
        ...tama/.vim/plugged/hologram.nvim/lua/hologram/init.lua:109: in function '_to_absolute_path'                                                                                                       
        ...tama/.vim/plugged/hologram.nvim/lua/hologram/init.lua:100: in function 'find_source'                                                                                                             
        ...tama/.vim/plugged/hologram.nvim/lua/hologram/init.lua:74: in function 'buf_generate_images'

Instead, the plugin should ignore them. The fix is to add if not source then return nil end inside hologram.find_source in lua/hologram/init.lua line 99.