res_pjsip_aoc: Don't assume a body exists on responses.

res_pjsip_aoc: Don't assume a body exists on responses.

When adding AOC to an outgoing response the code
assumed that a body would exist for comparing the
Content-Type. This isn't always true.

The code now checks to make sure the response has
a body before checking the Content-Type.

ASTERISK-21502

Change-Id: Iaead371434fc3bc693dad487228106a7d7a5ac76