fix(sql): standardize NULL handling of argmin/argmax

[jcrist]

This standardizes the NULL handling behavior of argmin/argmax to better match user expectations and the common implementation of the SQL min_by/max_by methods.

Previously some backends would jump through hoops to avoid returning NULL values from these methods, even if the value that minimized the column was NULL. We now no longer do this (and workaround the compatibility issues in a few backends that did this by default). Fixes #10193.