Added Nullable Support for runningDifference Function

Why it matters

This feature allows the runningDifference function to handle columns with Nullable types, enabling calculations on data that may contain null values without errors. It improves the function's robustness and usability when working with incomplete or optional data.

How to use it

Use the runningDifference function on columns of Nullable types as usual. The function now natively supports these types, so no additional changes or flags are required.