Standardize api.Height handling

[StephenButtolph]

Why this should be merged

By convention, to approximate the behavior of Unmarshal itself, Unmarshalers implement UnmarshalJSON([]byte("null")) as a no-op.

ref: https://pkg.go.dev/encoding/json#Unmarshaler

How this works

If null is passed in, then we return immediately.

How this was tested

This changed all of the existing ad-hoc tests into test vectors which (I think) are easier to see what exactly the expected behavior is (and what is being tested)

Need to be documented in RELEASES.md?

No

[iansuvak]

Thank you for the standardization fix and test improvements. I'll do vectorized tests from now on.