Fix gitgutter#utility#setbufvar for nonexistent buffers
Fix gitgutter#utility#setbufvar for nonexistent buffers

Use a default with `getbufvar` to not get an empty string.

Fixes:

> Error detected while processing function <SNR>163_on_stderr_nvim[1]..<lambda>10[1]..gitgutter#utility#setbufvar:
> line    1:
> E712: Argument of get() must be a List or Dictionary

I can trigger it using https://github.com/AndrewRadev/linediff.vim when
closing the windows / the tab.
Likely triggered with/through Neomake.

airblade/vim-gitgutter