Fix joaotavora/eglot#285: unbreak Elm language server which does use …

…:triggerCharacters

Only query completionProvider -> triggerCharacter information if the
server has provided it.  Elm's, and probaly other's, do not provide
it, which doesn't mean they don't support completion.

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

eglot.el

@@ -1958,10 +1958,11 @@ 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)