range_key in get fails with truthy Falses
range_key in get fails with truthy Falses

Previously, if used a .get(hash_key, range_key) where bool(range_key)
was False, the range_key wouldn't actually show up in the query to
DynamoDB because the get_identifier_map function would simply use
the truthy evaluation `if range_key`. By changing this to an
explicit `if range_key is not None`, this allows gets when a Python
value would otherwise evaluate to False, e.g., 0.

Villoid/PynamoDB,borgstrom/PynamoDB,pynamodb/PynamoDB,khwilson/PynamoDB,jlafon/PynamoDB,Evzdrop/PynamoDB