Said method has a check on `_lb` not being null, before accessing it.
However, since 0e5754a, there was an unconditional access, adding an
entry for the local node. Move this inside the if, so it is covered by
the null-check. The only caller is the api (probably nodetool), the
worst that can happend is that they get completely empty load-map if
they call too early during startup.
Fixes: #16617Closesscylladb/scylladb#16659
f4f724921c