Fix #285: unbreak Elm language server which does use :triggerCharacters

Only query :triggerCharacter information if the server has provided
it.  Elm's doesn't, apparently.

* eglot.el (eglot-completion-at-point): Check that completion
capability is a list before treating it like one.

eglot.el

@@ -1952,10 +1952,11 @@ is not active."
        :company-prefix-length
        (save-excursion
          (when (car bounds) (goto-char (car bounds)))
-         (looking-back
-          (regexp-opt
-           (cl-coerce (cl-getf completion-capability :triggerCharacters) 'list))
-          (line-beginning-position)))
+         (when (listp completion-capability)
+           (looking-back
+            (regexp-opt
+             (cl-coerce (cl-getf completion-capability :triggerCharacters) 'list))
+            (line-beginning-position))))
        :exit-function
        (lambda (comp _status)
          (let ((comp (if (get-text-property 0 'eglot--lsp-completion comp)