CatPipe.get: don't throw MissingObject; return None

CatPipe.get: don't throw MissingObject; return None

This will be more heavily used later and exceptions are notably more
expensive.  Further, a missing object isn't necessarily an error.
It's perfectly reasonable to attempt to check an object's existence
via get().

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>