fix(filetype.lua): always return a string in getline helper function (#17852)

Uses of `getline` in `filetype.lua` currently assume it always returns a
string. However, if the buffer is unloaded when filetype detection runs,
`getline` returns `nil`. Fixing this prevents errors when filetype
detection is run on unloaded buffers.

runtime/lua/vim/filetype.lua

@@ -21,7 +21,7 @@ end
 
 ---@private
 local function getline(bufnr, lnum)
-  return api.nvim_buf_get_lines(bufnr, lnum-1, lnum, false)[1]
+  return api.nvim_buf_get_lines(bufnr, lnum-1, lnum, false)[1] or ""
 end
 
 -- Filetypes based on file extension