accept the cached assembly when the public token of the given assembly is null.
accept the cached assembly when the public token of the given assembly is null.

This makes Assembly.Load calls with simple assembly names work.
Fixes #1199

mglodack/FAKE,rflechner/FAKE,MiloszKrajewski/FAKE,mglodack/FAKE,beeker/FAKE,MiloszKrajewski/FAKE,mglodack/FAKE,beeker/FAKE,beeker/FAKE,MichalDepta/FAKE,rflechner/FAKE,Kazark/FAKE,rflechner/FAKE,Kazark/FAKE,mglodack/FAKE,MichalDepta/FAKE,MiloszKrajewski/FAKE,Kazark/FAKE,MichalDepta/FAKE,rflechner/FAKE,MichalDepta/FAKE,MiloszKrajewski/FAKE,beeker/FAKE,Kazark/FAKE