Rolling index builds do not have buildUUIDs, this is only set for two-phase, or replicated, index builds.
However, when we issue the create index command we unconditionally generate a buildUUID that we end up logging, even if we do not use this uuid.
This can lead to confusing behavior where in some logs a rolling index build will have a buildUUID of null and in others it will have a buildUUID as if it were a replicated index build.