Currently we have no way to actually set one
of the following limits to 0 (meaning not usable):
- maxBuckets
- usageLimit
- bandwidthLimit
With having the field nullable,
NULL corresponds to the global default,
0 now actually 0 and
a set value determines a custom limit.
Change-Id: I92bb77529dcbd0881ae8368921be9d246eb0919e