Option to disable NULL_VALUE output processing

[rayokota]

Adds a per-instance toggle on Jsonata that controls whether the final Utils.convert_nulls(...) pass runs on the evaluation result, mirroring the upstream jsonata-java change in [dashjoin/jsonata-java#59][1] (commit fbb6878`).

  j = jsonata.Jsonata("$")                                                                                                                                                          
  j.set_output_convert_nulls(False)                                                                                                                                                 
                                                                                                                                                                                    
  j.evaluate(jsonata.Utils.NULL_VALUE)   # → Utils.NULL_VALUE  (JSONata null)                                                                                                       
  j.evaluate(None)                       # → None              (JSONata undefined)

Fixes #25