use tag branch name even when the branch has no namespace fixes #2201

[carlwoodhouse]

Description

As suggested in #2201 this fix uses the untrimmed branchname if a trim of the namespace prefix has resulted in an empty tag name, for branches with non namespaces eg 'somebranch" (#2201 (comment))

Related Issue

Resolves #2201

Motivation and Context

To allow prerelease tags for branches which aren't namespaced

How Has This Been Tested?

via a new unit test for the scenario(s), old ones are already covered.

Screenshots (if appropriate):

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[mergify]

Thank you @carlwoodhouse for your contribution!