Add support for SQL-compatible null handling to movingAverage query

This issue is for implementing #4349-compliant null-handling in movingAverage query.

Background:
movingAverage query (#6430) doesn't currently support the `druid.generic.useDefaultValueForNull=false` (This is noted in documentation). An exception will be raised by the query if this flag is set.

movingAverge query has null-handling built into it, which mostly based on the original "replace with default" convention but is a bit more flexible (Allows the user to choose null-aware Averager).

The issue names no implementation file or test; start with the movingAverage query and read issues #4349 and #6430 for the required null-handling behavior. Review the documentation note about the flag, then verify that movingAverage works with druid.generic.useDefaultValueForNull=false without raising an exception.

Written by the indexing model from the issue text.