res_pjsip: Avoid crash when contact uri is empty string

Asterisk will crash if contact uri is invalid, so contact_apply_handler
should check if the uri is NULL or empty.

ASTERISK-27393 #close
Reported-by: Aaron An
Tested-by: AaronAn

Change-Id: Ia0309bdc6b697c73c9c736e1caec910b77ca69f5