OK, here's my work around.

I peeked at the default value in an empty HTML field and it contains this:

<p><br data-mce-bogus="1"></p>

Once someone adds their own information in that HTML field, that default value is no longer present.

So in my business rule's Filter Conditions I'm just checking to ensure the 'data-mce-bogus' is NOT in the field.

That seems to work.