Add default "null" to JSON::Builder#field

[straight-shoota]

The method JSON::Builder#field(name, &) expects the block to append a value to the builder. If this doesn't happen (for example because the block is empty), the builder enters an invalid state.

This patch adds detection for this.

• It adds a default null value if the builder state still expects the name of an object property (which suggests the block didn't do anything to the builder).
• It raises on any other than ObjectState, because that indicates the block appended an incomplete value.

This could be even further enhanced and actually check the validity of the state stack. But that's hardly necessary.
The main use case is code that for some reason just forgets appending a property value in the block.
Defaulting to null makes that it still produces valid JSON output which can then be used to investigate the error if a value was expected instead of null.

Resolves #5218