fix: don't attach lsp to buffer with empty path

[IgorKhramtsov]

For some reason, on one of my projects, LSP tries to attach to a buffer with an empty path. Dart language server recognizes this as / root directory and starts to analyze the whole file system and eventually just freezes.
https://github.com/akinsho/flutter-tools.nvim/blob/271eec9edb0f1a2bf30ad449ec3b4eeb2c88af05/lua/flutter-tools/lsp/init.lua#L198
After adding some print statements for debugging, I found out it happens every time the floating window with diagnostic is open. However, there is no such buffer when I check the buffer list with :ls, so I'm not sure what exactly causes this. Anyway, check for an empty string is a nice addition to is_valid_path function.

[sidlatau]

Thanks, @IgorKhramtsov