Debug version of QCoreTextFontDatabase::fallbacksForFamily on Mac OS X 10.15(Beta)

#1. The root cause is missing `/System/Library/Frameworks/ApplicationServices.framework/Frameworks/CoreText.framework/Resources/DefaultFontFallbacks.plist` file in Mac OS X 10.15(Beta).

#2: After loading this file `DefaultFontFallbacks.plist` (from `qt5/qtbase/src/platformsupport/fontdatabases/mac/qcoretextfontdatabase.mm`)
into `fallbackDict`,  `fallbackDict` becomes `nil` on Mac OS X 10.15(Beta).

#3: `Q_ASSERT` raises SIGABRT signal in Debug build. 

I tested with official Qt binaries with maps.app example on Mac OS X 10.15(Beta), it crashes in the same place.