fixing None types in hashes

[slorello89]

Was investigating #562 when I discovered to my dismay that HashModels now fail if they encounter a None. The genesis of this is thedict method in pydantic which produced '' as the default value if the value for a string field was None, changed in pydantic 2.x, now it simply returns None if no default is specified. Now this wouldn't be a problem except for the fact that redis-py cannot handle a None value in HSET (fair enough).

Altering the behavior to exclude None type from HSET command, while still allowing you to define a default value for the field.

redis-om-py 0.2.x behavior: None types in a hash default to ''

redis-om-py 0.3.0 behavior: None type fails in a hash

new behavior: None is just None

[bsbodden]

LGTM!