sys/cbprintf_internal.h: Silence spurious warnings with -save-temps

Because Z_CBPRINTF_STATIC_PACKAGE_GENERIC()'s first argument can be a
static memory location and we have `-Wall`, which implicitly enables
`-Waddress`, we get the following warnings when you enable
`EXTRA_CFLAGS=-save-temps=obj`.

    $ west build -b qemu_cortex_m3 samples/hello_world -- \
          -DEXTRA_CFLAGS=-save-temps=obj
       :
       :
    zephyr/kernel/fatal.c: In function 'k_sys_fatal_error_handler':
    zephyr/kernel/fatal.c:45:1125: warning: the comparison will
    always evaluate as 'true' for the address of 'data' will never be NULL
    [-Waddress]
       45 |         LOG_ERR("Halting system");
          |
    In file included from zephyr/include/zephyr/logging/log_backend.h:9,
                     from zephyr/include/zephyr/logging/log_ctrl.h:10,
                     from zephyr/kernel/fatal.c:13:
    .../include/zephyr/logging/log_msg.h:94:17: note: 'data' declared here
       94 |         uint8_t data[];
          |                 ^~~~

The reason why you don't see this warning without the flag is that GCC
tracks tokens and suppress the warning if it's from a macro expansion.  You
can disable this feature by adding `-ftrack-macro-expansion=0`:

    west build -b qemu_cortex_m3 samples/hello_world -- \
          -DEXTRA_CFLAGS=-ftrack-macro-expansion=0

Because `-save-temps` generates .i files, all macros have been expanded and
the information has already been lost.  All GCC sees at the compilation
stage are the comparisons of static memory locations.

This commit replaces `buf != NULL` with a static inline function
`___is_null()` to silence the compiler.  By passing a static memory
location to a function, the compiler doesn't see the comparisons of a
static memory locations against literal values.  But it's still able to
optimize out in the later stage.

There is another way to silence it; By ignoring `-Waddress` with a pragma.
But its effect is the macro wide and it's too wide IMHO.  Thus, I've
decided to go with the inline function.

To add one more note: The name `___is_null()` is obviously too generic.
But let's have it here until someone finds it problematic.

This closes #51528.

Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>
(cherry picked from commit d51f8741)