Fix Visual guifg

vim/vim#14025 changed the default from `guifg=NONE` to
`guifg=LightGrey`, so we must now explicitly `guifg=NONE` in order to
unset it, so that syntax highlighting is preserved in visual mode.

colors/monokai.vim

@@ -38,14 +38,14 @@ let g:terminal_ansi_colors = repeat(values(s:ansi_hex), 2)
 function! s:highlight(group, fg, bg, ...)
   let l:attrs = a:0 ? a:1 : 'NONE'
   exe 'hi ' .. a:group .. ' cterm=' .. l:attrs .. ' gui=' .. l:attrs
-  if a:fg != ''
-    exe 'hi ' .. a:group .. ' ctermfg=' .. s:colors[a:fg][g:monokai_256_cterm]
-    exe 'hi ' .. a:group .. ' guifg=' .. s:colors[a:fg][2 + g:monokai_256_gui]
-  endif
-  if a:bg != ''
-    exe 'hi ' .. a:group .. ' ctermbg=' .. s:colors[a:bg][g:monokai_256_cterm]
-    exe 'hi ' .. a:group .. ' guibg=' .. s:colors[a:bg][2 + g:monokai_256_gui]
-  endif
+
+  let l:fg = get(s:colors, a:fg, ['NONE', 'NONE', 'NONE', 'NONE'])
+  exe 'hi ' .. a:group .. ' ctermfg=' .. l:fg[g:monokai_256_cterm]
+  exe 'hi ' .. a:group .. ' guifg=' .. l:fg[2 + g:monokai_256_gui]
+
+  let l:bg = get(s:colors, a:bg, ['NONE', 'NONE', 'NONE', 'NONE'])
+  exe 'hi ' .. a:group .. ' ctermbg=' .. l:bg[g:monokai_256_cterm]
+  exe 'hi ' .. a:group .. ' guibg=' .. l:bg[2 + g:monokai_256_gui]
 endfunction
 
 " General