if gdiplus.dll cannot be loaded, return NULL from wxGraphicsContext::Create() instead...

if gdiplus.dll cannot be loaded, return NULL from wxGraphicsContext::Create() instead of silently pretending to render the output, but do nothing

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@54233 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775