Time - fix type error where TooltipButton text could be undefined

[shleewhite]

📌 Summary

If merged, this PR would resolve a type error where the TooltipButton text could be undefined. Opted to just return '' if the date is invalid or if it cannot be converted into a UTC ISO format because other functions like rangeIsoUtcString and the hds-format-time helper return an empty string if it is invalid.

Other options considered

• use the JavaScript date methods as a fallback: not used because the case where this happens most likely is when it is an invalid date, so this would not work
• update the TooltipButton to let the text argument be optional: not used because this would have much larger consequences and we'd rather ensure tooltips always have text.

🔗 External links

Jira ticket: HDS-4517

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👀 Component checklist

• Percy was checked for any visual regression
• A changelog entry was added via Changesets if needed (see templates here)

💬 Please consider using conventional comments when reviewing this PR.

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[KristinLBradley]

Looks good!