Fix type signature for tensor names

[jleibs]

Following up on #631 fixed the type signature.

All other handling seemed to be done already:

import rerun as rr
import numpy as np

rr.init("tensor_test", spawn=True)

T = np.random.rand(10, 10, 10)
rr.log_tensor("tensor_test", T, names=["foo", None, "bar"])

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)