fix: credential secrets: handle case where no default is set (#2411)

[g-linville]

for #2411

This adds a check so that we do not print (default: <default>) for fields in the secret that have no default value set.

Checklist

• [x] The title of this PR would make a good line in Acorn's Release Note's Changelog
• [x] The title of this PR ends with a link to the main issue being address in parentheses, like: This is a title (#1216). Here's an example
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• [x] Commits follow contributing guidance
• [ ] Automated tests added to cover the changes. If tests couldn't be added, an explanation is provided in the Verification and Testing section
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