I have a few tags with spaces which render in the tag cloud as hyphens which is OK but the linked pages are always empty. I found the fix in tags/list.html where I had to change
{{ $tag := lower .Title }}
to
{{ $tag := lower .Title | urlize }}
And while I was improving the tags I removed the hyphen when a tag is displayed in the tag cloud by editing layouts/partials/widgets-tags.html using the advice here
<a href="/categories/{{ $name | urlize }}">{{ replace $name "-" " " }}</a>