Evaluate InputfieldRepeater::isEmpty() differently when inputfield is in $singleMode (FieldsetPage)

[Toutouwai]

Short description of the enhancement

FieldsetPage uses InputfieldRepeater for its inputfield, but it sets a special $singleMode that affects several things.

Something else that it would be good for $singleMode to do is change the behaviour of InputfieldRepeater::isEmpty().

A normal repeater can have zero items or many items, and so it makes sense that in normal mode isEmpty() will return a value based on the count of the repeater items.

But FieldsetPage always has one item, so InputfieldRepeater::isEmpty() doesn't do anything useful for FieldsetPage - it always returns false. When $singleMode is true it would be better if InputfieldRepeater::isEmpty() returned true when none of the fields within the FieldsetPage are populated and false when any of them are populated.

That would allow the "when blank/populated" visibility settings for the inputfield to work, as they currently don't work for FieldsetPage. Related forum topic: https://processwire.com/talk/topic/22666-visibility-fieldset-page/