When in doubt, search in the Design And Reference Guide for the section "Member Expansion Where Clause". There we find the following:
A#Root.Children(Text5 <> '') filters out the Members that have an empty value for the Text5 property.
I suspect the issue you might have is that Text properties vary by Scenario type and by Time, and GetMembersUsingFilter has no way to specify those, so maybe you can't use that type of filter with that call. In that case you might have to do it yourself by removing the clause, retrieving a more generic list of MemberInfo objects, then looping through the list calling their GetMemberInfoForProperty() method with the necessary parameters to look up that property.