patch to avoid a NullPointerException

I have observed a NullPointerException raised in PGobject.equals().
The attached patch checks for a null value and avoids raising an
exception.

I am working with an application which makes extensive use of
user-defined types, particularly enums, and also uses EclipseLink.  I
found it necessary to create EclipseLink converters to convert from
the Java representation of the user-defined types to a PGobject so
that PgJDBC sends the correct type information to the PostgreSQL
server.  It appears to be necessary to create an PGobject instance
with the correct type label and a null value in order to store a NULL
in a column with a user-defined type.  However, this results in a
NullPointerException when EclipseLink tests the two PGobject instances
for equality.

-Derrick