drivers: modem: initialize variables to avoid warn

The variables 'first' and 'next' in function 'stats_buffer_list_first()'
and 'stats_buffer_list_next()' were potentially used uninitialized.

Depending on the compiler and target architecture, this can lead to
different behavior, including warnings or errors when using strict
warning flags.

By initializing these pointers to 'NULL', we ensure consistent and
expected behavior across all toolchains and configurations.

Signed-off-by: Fabian Kainka <kainka@cognid.de>