macOS: Crash when accessing printer while running with "minimal" platform plugin

When running with the "minimal" plugin on macOS, accessing even a default printer triggers a crash in QCocoaPrinterSupportPlugin::create(QString const&).

While at first it may seem strange to use this particular combination, the bug was discovered in test code which run flawlessly unattended in Windows and Linux environments. For such purposes, it is fair to expect this to work on macOS, too.

The cocoa printsupport plugin seems to be the only one which uses the platform plugin, unlike the cups and windows printsupport plugins.