distinguish between string contraint name and defined

distinguish between string contraint name and defined

Take _NONE_NAME into account as a valid constraint name
and don't skip these constraints or consider them to be unnamed.
Thanks to typing this also revealed that previous batch versions
were also keying "_NONE_NAME" constraints as though they were named.

Fixed regression for 1.10.0 where :class:`.Constraint` objects were
suddenly required to have non-None name fields when using batch mode, which
was not previously a requirement.

Change-Id: If4a7191a00848b19cb124bc6da362f3bc6ce1472
Fixes: #1195