Add support for testValue definitions in nested props only

[zchsh]

This PR removes the need to defined an empty base testValue: {} for object properties. An effect of this change is that object properties will now return an empty object as their testValue, even if it is not defined.

We may or may not want to make this change, just suggesting it as defining an empty object as a testValue at multiple levels in hashicorp/react-components#154 felt odd.

The approach taken here is:

• If testValue on an object prop is not explicitly set, then we'll default it to {}. This way, we don't have to explicitly set the base empty object testValue: {} at every level of a complex object prop.
• The current behaviour is still possible - it can be opted into by explicitly setting testValue: undefined on the "object" prop. This only needs to be done once, at the "top level" of the tree.

[hashicorp-cla]

All committers have signed the CLA.