Fix build on Winsdows
Fix build on Winsdows

Use Shiboken::AutoDecRef::isNull() to fix MSVC error:
pyside2\libpyside\pysidesignal.cpp(409): error C2593: 'operator ==' is ambiguous
qtbase\include\qtcore\../../src/corelib/tools/qpoint.h(329): note: could be 'bool operator ==(const QPointF &,const QPointF &)'
qtbase\include\qtcore\../../src/corelib/tools/qpoint.h(149): note: or       'bool operator ==(const QPoint &,const QPoint &)'
...

Amends change 8726914a418462194a3fbd8187728f411fb70988.

Task-number: PYSIDE-79
Change-Id: I3cae3d9b0192a96ef718e4c4a151ff7ca2949858
Reviewed-by: Christian Tismer <3f32342346a37828fd9c8841ea4579d28b46cf08@stackless.com>

qtproject/pyside-pyside,qtproject/pyside-pyside,qtproject/pyside-pyside,qtproject/pyside-pyside,qtproject/pyside-pyside