Bluetooth: Mesh: Ensure proxy_send's attribute is always set
Bluetooth: Mesh: Ensure proxy_send's attribute is always set

Initializes proxy_send's attr pointer, which in theory could be
uninitialized if neither gatt proxy or PB GATT is enabled while proxy.c
is enabled.

This scenario is not really possible with the current kconfig structure,
but is reported as a potential error by Coverity, and should be fixed
for future proofing purposes.

Fixes #36314.

Signed-off-by: Trond Einar Snekvik <cbf21a7cf058d7ce8e74b0726695057d761f6ae9@nordicsemi.no>

galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr