Ensure RemoteBinding.clientBindUrl is defaulted if ProxyFactory is not created via JNDI Registrar

Current implementation defaults a blank or null clientBindUrl in the JNDI Registrar, when creating Proxy Factories. However, Proxy Factories may also be created directly, and handlers need appropriate defaults in this case. Extract out the relevant defaulting code such that it'll be accessible to both the JNDI Registrar and Handler constructors