Package schema need not allow null values

[jpmckinney]

In releases, null has a particular meaning in terms of merging. In packages, null has no special meaning and, in general, OCDS prefers to omit fields rather than include fields with null values.

This would be a backwards-incompatible change, so would be a consideration for OCDS 2.0. As the presence of null values is not a serious issue, we may chose to continue to allow them, in order to lower the implementation cost of the 2.0 upgrade.

See #677 for prior discussion.

[duncandewhurst]

I think it makes sense to remove them. Otherwise, the meaning of null values would be inconsistent between packages and releases. By providing a strict schema in OCDS 1.2, fewer publishers are likely to need to do this as part of an upgrade to 2.0.