res_pjsip/configuration: Fix a variety of default value problems

res_pjsip/configuration: Fix a variety of default value problems

This patch fixes some bad default value handling in the following
settings:

* The 'message_context' and 'accountcode' settings are not mandatory. As
  such, we can allow their stringfield values to be empty.
* The 'media_encryption' setting applies a default value of 'none' to
  the setting, which it then can't parse or understand. Since the value
  is documented to be 'no', this will now apply that as the default
  value.

Change-Id: Ib9be7f97a7a5b9bc7aee868edf5acf38774cff83