Fix handling of nulls passed via context

[rayokota]

This is a port of the corresponding fix in the Python implementation of JSONata, see this.

The fix allows a null obtained from the context to be properly handled, so that the following two expressions return the same result (where a is undefined):

     jsonata("$split(a, '-')").evaluate(Map.of());

     jsonata("a ~> $split('-')").evaluate(Map.of());