Allowance of null schema types is inconsistent

[abargnesi]

Not all optional properties can be set as a null type. This should be consistent for all because I believe the semantics of null equal that of omission.

This is with respect to one of our stated design principles:

Properties that allow a null value can be omitted.

We should rewrite it as:

Optional properties can either be null or omitted.

Offending properties:

• graph.label
• graph.type
• node.label
• edge.id
• edge.relation
• edge.label

[wshayes]

Handled with Version 2 of the JGF schema definition.