feat: Add getStringOrDefault method to FieldValue

[PhongChuong]

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Fixes #3179 ☕️

[shollyman]

Should we exercise this in the tests using a null string rather than sentinel string value? Similarly, do we need to do this for all the typed getters?

[PhongChuong]

/gcbrun

[PhongChuong]

Added comment suggestions.
And as discussed offline, we will only provide getStringOrDefault for String types for now. Other types can be added as if there are user requests for it.

[PhongChuong]

Thanks for the review.