Eglot: fix null scopeUri regression in workspace/configuration

Eglot: fix null scopeUri regression in workspace/configuration

* lisp/progmodes/eglot.el (eglot-handle-request):
Commit 1a2d603bb3938ff68ed1a5412d131b41efd40a24 changed
`eglot--uri-to-path' to return a nil uri untouched.  (Before,
`url-unhex-string' turned the parsed all-nil uri record into the empty
string.)

A nil return value must now be handled in the caller, do that for the
workspace/configuration handler to avoid an uncaught error.