Such rebuild has no read_from replica, but we know the tablet size will be 0.
If we don't, stats will be incomplete until the next refresh.
This is important for test cases which do removenode or replace while
all replicas are down. So for example test_replace from
test_tablets_removenode.py, which uses RF=1 and replaces a node.
Without this, the test waits for 60s needlessly after the first round
of rebuilding migrations before scheduling more migrations. This can
cause the test to time out.
Fixes#28115Closesscylladb/scylladb#28121
3fb7719277