Fix NRE when returning null from a scheme handler factory (#1792)
Fix NRE when returning null from a scheme handler factory (#1792)

Regression in 49.0 release.  Prevented use of default CEF handler when
using a custom scheme handler.
jamespearce2006/CefSharp,jamespearce2006/CefSharp,jamespearce2006/CefSharp,Livit/CefSharp,jamespearce2006/CefSharp,Livit/CefSharp,Livit/CefSharp,jamespearce2006/CefSharp,Livit/CefSharp