Unbreak elm language server which does use :triggercharacters

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.

GitHub-reference: fix https://github.com/joaotavora/eglot/issues/285