Replace zero length strings with NULL pointers.
Replace zero length strings with NULL pointers.

This avoids some compiler warnings. The type of the string literal is a
const char pointer, while the send_cmd() function expects an unsigned
char pointer.

Poltsi/libdivecomputer-vms,henrik242/libdivecomputer,henrik242/libdivecomputer,glance-/libdivecomputer,glance-/libdivecomputer