I think this should be&where=`[{"myTemplateVar:!= ""}]`
&where=`[{"myTemplateVar:!= ":""}]`
[[+myTemplateVar:notempty=`<li ...>[[+myTemplateVar]]</li>`]]
[[+myTemplateVar:isnot=``:then=`<li ...>[[+myTemplateVar]]</li>`]]
I think this should be&where=`[{"myTemplateVar:!= ""}]`&where=`[{"myTemplateVar:!= ":""}]`