I have tried using Plugin:ifEmpty in WebSphere Portal 8.5 and they worked fine with Rich Text Editors.
This was the code that I used in the presentation template:-
[Plugin:ifEmpty value="[Element context='current' type='content' key='richText']"]Empty![/Plugin:ifEmpty]
[Plugin:ifNotEmpty value="[Element context='current' type='content' key='richText']"]Not Empty![/Plugin:ifNotEmpty]
It worked pretty well and responded appropriately.
May be you need to update to the latest fix pack. The other possibility would be that you inadvertently put some value in the rich text editor in the authoring template and all contents created from it took this value. Check the source of the rich text editor to make sure that it is empty.