Sample configs: Fix pjsip.conf syntax error.

It is valid for a config file to be empty or contain only comments, but
not valid for a config value to be set when no uncommented context
exists.  This caused an error to be loged numerous times during start
when loading the default pjsip.conf.

Change-Id: Icf3b0d69b4ecb6e935eecd43c99ed8b32a5a1cf6

configs/samples/pjsip.conf.sample

@@ -835,7 +835,7 @@
                            ; This option must also be enabled in the system
                            ; section.
                            ; (default: no)
-suppress_q850_reason_headers =
+;suppress_q850_reason_headers =
                            ; Suppress Q.850 Reason headers for this endpoint.
                            ; Some devices can't accept multiple Reason headers
                            ; and get confused when both 'SIP' and 'Q.850' Reason