fix(util): error when filetype is nil

[doums]

Hi, on latest nvim v0.11.0-dev-1190+gc12be1249f this PR fixes the following issue, preventing Copilot to work

Error executing vim.schedule lua callback: ...e/.local/share/nvim/dev/copilot.lua/lua/copilot/util.lua:143: bad argument #1 to 'gsub' (string expected, got
nil)
stack traceback:
        [C]: in function 'gsub'
        ...e/.local/share/nvim/dev/copilot.lua/lua/copilot/util.lua:143: in function 'get_language_id'
        /usr/share/nvim/runtime/lua/vim/lsp/client.lua:1030: in function '_text_document_did_open_handler'
        /usr/share/nvim/runtime/lua/vim/lsp/client.lua:1049: in function '_on_attach'
        /usr/share/nvim/runtime/lua/vim/lsp/client.lua:631: in function ''
        vim/_editor.lua: in function <vim/_editor.lua:0>

[doums]

closing, dup /pull/340

[folke]

Has been fixed in core in the meantime, so should be ok with tonight's nightly