init: set init_entry.dev to NULL

[PR#51217](https://github.com/zephyrproject-rtos/zephyr/pull/51217

)
changed the `init_entry` structure, but `Z_INIT_ENTRY_NAME` (called
by `SYS_INIT` or `SYS_INIT_NAMED`) does not initialize all of the
members of the struct, leading to errors when building with
`-Werror=missing-field-initializers`.

Change the macro to initialize the `dev` member to `NULL` so that
all members of the struct are initialized.

Signed-off-by: Paul Fagerburg <pfagerburg@google.com>