Make `convertObjectProps` correctly handle string arrays

What does this MR do?

Fixes a bug in the convertObjectProps utility function (used by ever-popular convertObjectPropsToCamelCase and convertObjectPropsToSnakeCase) so that it correctly handles string arrays.

Previously, something like this would throw an error:

convertObjectPropsToSnakeCase({ milestones: ['1.2', '1.3']})

This is because convertObjectProps would attempt to convert the "property names" of the array, which in this case are "0" and "1". The convertToSnakeCase utility doesn't handle property names with numbers (see the regex it uses here), so it throws an error when trying to convert these properties. (This issue should also be addressed, probably in a separate MR. Update: Opened an MR for this here: !33528 (merged).)

The fix is to avoid running the conversion function on the property names if the object being converted is an array.

---

I'm a little bit surprised we haven't run into this before 🤷 Maybe our API doesn't have many endpoints that accept an array of just strings?

Context

I ran into this issue when trying to use convertObjectPropsToSnakeCase with a Release API payload, which includes a property that looks like this:

{
   milestones: ["12.1", "12.2"]
}

Here's the MR: !33434 (merged)