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

res/res_pjsip/pjsip_configuration.c

@@ -700,7 +700,7 @@ static int media_encryption_handler(const struct aco_option *opt, struct ast_var
 
 static const char *media_encryption_map[] = {
 	[AST_SIP_MEDIA_TRANSPORT_INVALID] = "invalid",
-	[AST_SIP_MEDIA_ENCRYPT_NONE] = "none",
+	[AST_SIP_MEDIA_ENCRYPT_NONE] = "no",
 	[AST_SIP_MEDIA_ENCRYPT_SDES] = "sdes",
 	[AST_SIP_MEDIA_ENCRYPT_DTLS] = "dtls",
 };
@@ -1926,8 +1926,8 @@ int ast_res_pjsip_initialize_configuration(void)
 	ast_sorcery_object_field_register(sip_sorcery, "endpoint", "g726_non_standard", "no", OPT_BOOL_T, 1, FLDSET(struct ast_sip_endpoint, media.g726_non_standard));
 	ast_sorcery_object_field_register_custom(sip_sorcery, "endpoint", "redirect_method", "user", redirect_handler, NULL, NULL, 0, 0);
 	ast_sorcery_object_field_register_custom(sip_sorcery, "endpoint", "set_var", "", set_var_handler, set_var_to_str, set_var_to_vl, 0, 0);
-	ast_sorcery_object_field_register(sip_sorcery, "endpoint", "message_context", "", OPT_STRINGFIELD_T, 1, STRFLDSET(struct ast_sip_endpoint, message_context));
+	ast_sorcery_object_field_register(sip_sorcery, "endpoint", "message_context", "", OPT_STRINGFIELD_T, 0, STRFLDSET(struct ast_sip_endpoint, message_context));
-	ast_sorcery_object_field_register(sip_sorcery, "endpoint", "accountcode", "", OPT_STRINGFIELD_T, 1, STRFLDSET(struct ast_sip_endpoint, accountcode));
+	ast_sorcery_object_field_register(sip_sorcery, "endpoint", "accountcode", "", OPT_STRINGFIELD_T, 0, STRFLDSET(struct ast_sip_endpoint, accountcode));
 
 	if (ast_sip_initialize_sorcery_transport()) {
 		ast_log(LOG_ERROR, "Failed to register SIP transport support with sorcery\n");