Avoid ArgumentNullException under WinRT

The WinRT implementation of GetPropertyFromDeclaredType() will return
null is no suitable property is found, or the property is static, if
unchecked this cases an unhandled ArgumentNullException in
ShouldInject(). This is particularly event if binding a singleton type
with a static Instance property.
Merge pull request #1 from andycb/master

Avoid ArgumentNullException under WinRT