[feature request] allow null for sort_by() as like as sort()

It show that sort() is allow null
http://jmespath.org/specification.html#sort

However I found that sort_by() will throw exception if expression return null

Start by reading the JMESPath sort() specification linked in the issue and comparing the sort() and sort_by() entry points. Reproduce the exception when sort_by() receives an expression returning null, then verify that null is handled consistently with sort() without changing other sorting behavior.

Written by the indexing model from the issue text.