qmake: skip empty parts when splitting strings in some more places

... and make it explicit where we can't do that for semantical or
backwards compat reasons.

most urgently, this fixes an assertion failure when $QMAKEFEATURES
contains empty paths (e.g., due to a trailing semicolon).

notable observation: QByteArray::split() has no argument for the split
behavior (it always keeps empty parts).

Task-number: QTBUG-47325
Change-Id: I72d4b2e154a2ed1802cfa98fb4a5211a68e43231
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
(cherry picked from qtbase/7c87ffff9aa62883bf61efe1bf27704d479eace5)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>