Ensure timestamps are formatted correctly in product analytics tables

What does this MR do and why?

Fixes a bug introduced in #427217 (closed) where timestamp values were incorrectly being converted to delimited numbers.

We now first check if the value is a date string, and if so, return the value as-is. This ensures date strings are displayed as they were before the change introduced in #427217 (closed).

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard Product Analytics in a project by setting up the feature at Project > Analytics > Analytics dashboards. (be sure to generate some events)
3. Visit the visualization designer at Project > Analytics > Analytics dashboards > Visualization designer.
4. Create a new visualization using the Data table visualization type.
5. Add a measure with some data.
6. Set the Events over time dropdown to group by a time dimension.
7. View the visualization and see the timestamp is incorrect.

Related to #439577 (closed)