Allow to set `neoterm_marker` to empty

[petobens]

Consider the following minimal init.vim file:

let $DOTVIM = expand('$HOME/.config/nvim')

set runtimepath+=$DOTVIM/bundle/repos/github.com/kassio/neoterm
filetype plugin indent on

let g:neoterm_marker = ''

If I now open nvim and run :Topen then neoterm doesn't open. If I remove the marker line everything works as expected but the terminal buffer has ;#neoterm-1 (which I want to remove).

[petobens]

Thanks!