lib: document the possibility of using NULL with `lldpctl_atom_set_str()`
lib: make it safe to pass NULL to `lldpctl_atom_set_str()`

Even if we don't allow such a thing.
lib: bump versioning

Even if the ABI stays the same, the ability to pass NULL is an interface
change (backward compatible). We don't want new versions of lldpcli to
interact with an old version of the library.
debian: update symbol versioning for incompatibility of `lldpctl_atom_set_str()`