Fix NaN:NaN error while aggregating duration column

[Aritra8438]

Fixes #2963

Technical details

This PR:

• Downcast Interval to string. (which eventually fixes the NaN:NaN part).
• Adds some tests for the same.

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[Aritra8438]

Hi, @mathemancer.
This PR solves the interval error!

On contrary to our guess, it was not a frontend issue (atleast, not wrt currently what we are doing in frontend).

[mathemancer]

Good catch! Thanks for this!