Fix #851: Prevent empty Diagnostic tags vector hiding main fontification

* eglot.el (eglot-handle-notification): Require that the resulting
list of faces is non-empty and that each face corresponds only to a
known tag.

For unknown tags, we don't pass any additional face information to
Flymake, and instead expect it to make the appropriate overlay with
the "severity" property of the Diagnostic.

Co-authored-by: João Távora <joaotavora@gmail.com>

#851: joaotavora/eglot#851

lisp/progmodes/eglot.el

@@ -1866,11 +1866,11 @@ COMMAND is a symbol naming the command."
                         (current-buffer) beg end
                         (eglot--diag-type severity)
                         message `((eglot-lsp-diag . ,diag-spec))
-                        (and tags
-                             `((face
-                                . ,(mapcar (lambda (tag)
-                                             (alist-get tag eglot--tag-faces))
-                                           tags)))))))
+                        (when-let ((faces
+                                    (cl-loop for tag across tags
+                                             when (alist-get tag eglot--tag-faces)
+                                             collect it)))
+                          `((face . ,faces))))))
            into diags
            finally (cond (eglot--current-flymake-report-fn
                           (eglot--report-to-flymake diags))