Remove nonzero requirement for unmarshaling influxdb.ID

The largest pain for me developing within the influxdb ecosystem is having to dance around zero values of influxdb.ID creating errors. We Either have to write custom unmarshalers (less than less than ideal), or use a different type altogether and either alias the ID, use a string, or use pointer semantics.

Ideally this behavior would belong to a Valid method that is called at will so the valid/invalid opinion established istn't forced on every consumer.

Start by locating the definition of influxdb.ID and its unmarshaling behavior in the repository. Trace where zero values are rejected and identify the existing validation entry point, then determine how tests cover unmarshaling and validity. Done means zero-valued IDs can be unmarshaled without forced errors while explicit validity checking remains available.

Written by the indexing model from the issue text.