fix __asdf_traverse__ for non-tagged object

[braingram]

Description

This PR adds a check for a _tag attribute before attempting to fetch schema for a node during NodeSchemaInfo tree traversal.

The current code assumes that any object implementing __asdf_traverse__ also has a _tag attribute which contains a valid tag.

"duck typing" was used here (instead of checking for isinstance(Tagged) as roman_datamodels defines non-Tagged objects that contain valid _tag attributes.

Fixes #1738

The weldx failure is unrelated (caused by a new pandas version) the roman_datamodels failure is addressed in: #1748

Checklist:

• pre-commit checks ran successfully
• tests ran successfully
• for a public change, a changelog entry was added
• for a public change, documentation was updated
• for any new features, unit tests were added

[braingram]

A few unrelated errors are seen in downstream:

• the weldx errors are due to a new pandas version
• the [specutils error] is due to a failure to fetch iers data
• the [stdatamodels error] is due to a crds update

[braingram]

Thanks again! I made a minor addition to the comment before the pass to include some details from the discussion here. It will hopefully serve as a reminder for future me (or whomever) encounters the pass in the future.