Empty diagnostics send invalid URI

[tomaskallup]

Describe the bug
When a file has diagnostic errors and they all get resolved, the server gives invalid URI (missing file:/ prefix).

To Reproduce

1. Open any c3 source file
2. Cause an error (for example remove ;)
3. Save
4. Fix the error
5. Save
6. Diagnostics with invald URI should be sent from the LSP

Expected behavior
The URI should be correct and error should just disappear.

Screenshots

Desktop (please complete the following information):

• OS: NixOS
• Editor: neovim

Additional context
I tried fixing this myself, but this required moving the clearOldDiagnostics under the Server struct so I'm not sure if it's the right way to do this (I have pretty much zero experience in Go).

I have my fix in a commit tomaskallup@489f24e, tested it after rebuilding and everything seemed fine.
The fix is tiny, so if it's correct, feel free to just commit it here, I could create a PR, but I would have to cherry-pick it into a fresh branch.

[pherrymason]

Thank you @tomaskallup !