fix(lsp): "Failed to load the import map"

[chibat]

I believe it is related to denoland/vscode_deno#718

The following reproduction procedure fails to load import map by deno lsp.

$ mkdir -p my-project/.vscode
$ echo '{ "deno.enable": true }' > my-project/.vscode/settings.json
$ cd my-project/
$ code .

open .vscode/settings.json(start deno lsp)

We get the following error message.

Failed to load the import map at: file:///home/chiba/my-project. [Is a directory (os error 21)]

I have added a check for empty characters.

[chibat]

Sorry. I will check the test error.

[kitsonk]

LGTM