Fix: Handle None Namespace Case Properly

[fahreddinozcan]

Currently, the below snippet upserts a vector to namespace None. We're now handling it to upsert to the default namespace.

index = Index.from_env()

vec = [1] * 1536

res = index.upsert(vectors=[{"id": "1", "vector": vec}], namespace=None)

[linear]

DX-1331 upstash-vector python sdk inserts to "None" namespace

[CahidArda]

can we add a test?