Apparently, a PreKeyBundle does not need to contain a one-time PreKey. (See the X3DH spec, 3.3 and 4.2 especially).
Including a PreKey in the bundle is certainly recommended. However, if the key server runs out of PreKeys for Bob, with PreKeys being optional, Bob can still be reachable by others trying to start sessions with him.
Apparently, a PreKeyBundle does not need to contain a one-time PreKey. (See the X3DH spec, 3.3 and 4.2 especially).
Including a PreKey in the bundle is certainly recommended. However, if the key server runs out of PreKeys for Bob, with PreKeys being optional, Bob can still be reachable by others trying to start sessions with him.