Coc.nvim: diagnostic.enableHighlightLineNumber adds an empty signcolumn

[alex-popov-tech]

Result from CocInfo

## versions

vim version: NVIM v0.4.2
node version: v13.6.0
coc.nvim version: 0.0.74-c57062d0da
term: iTerm.app
platform: darwin

## Messages
Error detected while processing /Users/alexanderpopov/.config/nvim/plugins.vim:
line  200:
E117: Unknown function: coc_explorer#command#generate
E15: Invalid expression: coc_explorer#command#generate({ 'toggle': v:true, 'file-child-template': '[git | 2] [selection | clip | 1] [indent][icon | 1] [filename growRight 1 omitCenter 1][modified]', 'file-child-labeling-template': '[fullpath][size][modified][readonly]', }, '/root/path')
15 fewer lines
"firmware/firmware.ino" 107L, 3764C written

"firmware/firmware.ino" 107L, 3766C written
## Output channel: explorer

Describe the bug

When I turn off sign column for diagnostic in coc-settings.json https://github.com/alex-popov-tech/.dotfiles/blob/master/nvim/coc-settings.json#L6
And set signcolumn to auto https://github.com/alex-popov-tech/.dotfiles/blob/master/nvim/settings.vim#L58
And I open a file with diagnostic errors/warnings

Expected result - i expect to see no signcolumn And highlighted line numbers for these lines
Actual result - there is empty signcolumn from coc https://share.getcloudapp.com/wbumr2yZ

Reproduce the bug

We will close your issue when you don't provide minimal vimrc and we can't
reproduce it

You can use my dotfiles to reproduce, here is coc.nvim part - https://github.com/alex-popov-tech/.dotfiles/blob/master/nvim/plugins.vim#L115-L217 and here is coc settings https://github.com/alex-popov-tech/.dotfiles/blob/master/nvim/coc-settings.json#L6-L7

Also reproduced by @fannheyward

Related to https://github.com/neoclide/coc.nvim/issues/1553

[chemzqm]

The line number highlight requires signcolumn exists, it's behavior of neovim.