Avoid usage of empty user name and password as UserPasswordCredentials.

A number of Git wizards provided empty strings (not nulls) as the user
name and the password in case the user has not specified these values
in the credentials fields on corresponding wizard pages.
That prevented to use credentials newither saved earlier in the secure
store nor login dialog.
Fix EclipseAuthenticator to support NTLM authorization scheme.

The existing default authenticator (EclipseAuthenticator) was
designed to support only requests from proxy hosts requiring
authentication. The change allows to support requests from
server sites if their requesting scheme is NTLM. That presumes
usage of the default credential provider inside of the default
authenticator (EclipseAuthenticator).

If the user has specified the user name and password credentials
in the clone wizard, the default authenticator is temporary
replaced (and then restored back) with an instance providing
the specified credentials.