Check that the request payload data is valid json.

By adding `force=True` to request.get_json() the mime type is ignore. If the data is not valid json the method will return a `BadRequestError` we catch that and throw our own error with a clear error message "Invalid JSON supplied in POST data".
If the json is valid return the json data or an empty dict if None is passed in.

This PR improves the error messages if the json is invalid, previously, the error message was "None object type" message which is not very helpful.

app/v2/notifications/post_notifications.py

@@ -3,7 +3,6 @@ import functools
 
 from flask import request, jsonify, current_app, abort
 from notifications_utils.recipients import try_validate_and_format_phone_number
-from werkzeug.exceptions import BadRequest
 
 from app import api_user, authenticated_service, notify_celery, document_download_client
 from app.celery.letters_pdf_tasks import create_letters_pdf, process_virus_scan_passed
@@ -58,6 +57,7 @@ from app.v2.notifications.notification_schemas import (
     post_letter_request,
     post_precompiled_letter_request
 )
+from app.v2.utils import get_valid_json
 
 
 @v2_notification_blueprint.route('/{}'.format(LETTER_TYPE), methods=['POST'])
@@ -173,15 +173,6 @@ def post_notification(notification_type):
     return jsonify(resp), 201
 
 
-def get_valid_json():
-    try:
-        request_json = request.get_json(force=True)
-    except BadRequest:
-        raise BadRequestError(message="Invalid JSON supplied in POST data",
-                              status_code=400)
-    return request_json
-
-
 def process_sms_or_email_notification(*, form, notification_type, api_key, template, service, reply_to_text=None):
     form_send_to = form['email_address'] if notification_type == EMAIL_TYPE else form['phone_number']