Fixes ambiguous cli output between "None" and NoneType

Print hyphen(-) in python client output when attribute value is not
set(NoneType) so that it is distinguishable between "None" string
and NoneType value.

Cherry-Pick: I5ec0f39f13aca45fa95ddb59472c20207c63bf91
Change-Id: I8a907e7453df711605d28bb97492f747adc282e0
Closes-Bug: #1266414