bluetooth: conn: Fix compiler warning

When compiling conn.c using arm-none-eabi-gcc version 11.3.1 20220712
with the -Wmaybe-uninitialized flag a warning is emitted due to
pending_no_cb not being initialized. I'm not sure if initializing it to
NULL is the "correct" fix, but it's certainly not any worse then it being
uninitialized, and it fixes the warning.

Signed-off-by: Dennis Grijalva <dennisgrijalva@meta.com>