Polish type prediction for generic factory methods

 - Now avoiding NullPointerExceptions in GenericTypeResolver's
   resolveReturnTypeForGenericMethod() in case the supplied ClassLoader
   is null.
 - AutowireUtils.resolveReturnTypeForFactoryMethod() now properly
   asserts that the supplied ClassLoader is not null.
 - Fixed copy-n-paste errors in Javadoc for
   AutowireUtils.resolveReturnTypeForFactoryMethod().

Issue: SPR-10411