Broken OfflineResolver and VerifySignature samples

[Nick Sydenham]

These two sample classes appear to be broken when trying to verify a 
signature that doesn't have an optional URI attribute on the Reference 
element.

In OfflineResolver.engineCanResolve the first java statement (line 98) 
should be:

if (uri == null) {
        return false;
}
However, adding this line means that you then get an 
"XMLSignatureException: The Reference for URI  has no XMLSignatureInput" 
exception when the VerifySignature sample is run.

Therefore, is the bug in the VerifySignature sample or the base library 
trying to check for URI's that don't exist?