res_pjsip/res_pjsip_callerid: NULL check on caller id name string

It's possible for a name in a party id structure to be marked as valid, but the
name string itself be NULL (for instance this is possible to do by using the
dialplan CALLERID function). There were a couple of places where the name was
validated, but the string itself was not checked before passing it to functions
like 'strlen'. This of course caused a crashed.

This patch adds in a NULL check before attempting to pass it into a function
that is not NULL tolerant.

ASTERISK-25823 #close

Change-Id: Iaa6ffe9d92f598fe9e3c8ae373fadbe3dfbf1d4a