[REG 4.14 -> 4.15] Failed .pro file parsing, no code files in Projects view

1. Create a new project of type "Qt Quick Application - Empty" using the wizard.
   All defaults, i.e. qmake based.
2. Check the tree in "Projects" view.
   It only shows the root node and the .pro file. No .cpp, no .qml.
3. Check "Issues" view.
   It reads:

   D:\dev\src\qt-5.15.2\qtbase\mkspecs\features\toolchain.prf:76: error: Variable QMAKE_CXX.COMPILER_MACROS is not defined.
   :-1: error: Project ERROR: msvc-version.conf loaded but QMAKE_MSC_VER isn't set
   D:\Testprojekte\untitledxx\untitledxx.pro:-1: error: Error while parsing file D:\Testprojekte\untitledxx\untitledxx.pro. Giving up.
   

   Would have to set these myself? Why?

This used to work fine in Creator 4.14.

Found by Squish test tst_CSUP01.