test: cover empty tag sets when prompting for tenant tags

[NickJosevski]

Fixes #529

Adding tests only

The reported bug is already fixed, incidentally, and this adds only the missing regression coverage. Close #529 without merging if you'd rather not carry the tests.

What was actually wrong

tenant create failed interactively with please provide options to select from on any space without usable tag sets. The old AskTags helper in pkg/cmd/tenant/create/create.go flattened every tag set into one list and always raised a survey.MultiSelect:

canonicalTagName := []string{}
for _, tagSet := range tagSets {
    for _, tag := range tagSet.Tags {
        canonicalTagName = append(canonicalTagName, tag.CanonicalTagName)
    }
}
err = ask(&survey.MultiSelect{Options: canonicalTagName, ...}, &tags)

With no tag sets — or only empty ones — Options is empty and surveyext/select.go rejects it.

Why it no longer reproduces

#548 (feat: Extend tag sets and add environment tags, Nov 2025) replaced AskTags with selectors.Tags, which iterates each tag set and skips the empty ones. That landed after the versions in the report (2.15.7 and 2.18.0), so the fix has been shipping since ~2.19.0.

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