#374: data_validator.py, use 'get()' to access JSON dict value
#374: data_validator.py, wrap 'file upload' validation with 'None' test
#374: data_validator.py, revert some code from '.get()' to bracket notation
Merge pull request #381 from jeff1evesque/python_data_validator

#374: Adjust 'data_validator.py', 'XML file' case