BUG-582: eliminate useless null checks

These checks are not useful, as the fields checked have been verified to
be non-null. Also optimizes AbstractPathArgument.hashCode() by directly
referring to the type -- we assume storing the PathArgument alongside
with the class is not going to be used very often.

Change-Id: Id3f4d67819de8462801823880e482a480843fa1e
Signed-off-by: Robert Varga <rovarga@cisco.com>