Add variant for jsonPathString(Object object, String jsonPath, String defaultValue)

Currently if we are not able to find a value at the given jsonPath, we just return defaultValue. This implies that Null value support functionality can't be used by the customer: https://docs.pinot.apache.org/developers/advanced/null-value-support

Can we add another variant where we return "null" in such scenarios ?

cc @npawar

Start by locating the jsonPathString(Object object, String jsonPath, String defaultValue) implementation and any related tests in Apache Pinot. Add the requested variant for missing paths while preserving the existing default-value behavior. Done means both variants have tests covering a found value and an absent JSON path.

Written by the indexing model from the issue text.