fix(ai-help): don't allow empty questions

[fiji-flo]

Summary

Don't allow empty (or only white space) questions when editing a question and disable the submit button if empty / only white space.

[argl]

lgtm

[caugner]

@fiji-flo I think you need to update the onKeyDown handler of the ExpandingTextarea in line 170 as well:

yari/client/src/plus/ai-help/index.tsx

Lines 167 to 181 in 94f81a5

	onKeyDown={(event) => {
	if (event.key === "Enter" && !event.shiftKey) {
	event.preventDefault();
	if (canEdit && question) {
	gleanClick(`${AI_HELP}: edit submit`);
	setEditing(false);
	submit(
	question,
	message.chatId,
	message.parentId,
	message.messageId
	);
	}
	}
	}}