Make result_pattern optional for POST interactions

[Sophietje]

The current code requires a result_pattern to be defined for a POST interaction, even though the Knowledge Engine supports POST interactions with only an argument_pattern. This is caused by

knowledge-mapper/mapper/knowledge_mapper/knowledge_mapper.py

Line 103 in 6245b18

result_pattern=ki["result_pattern"],

where it always tries to read the result_pattern . If the result_pattern is not defined then it'll cause a runtime error. It would be best if we make this optional by first checking whether this field is defined, and if it is not defined then initialize it to null.