Decide and implement default values for datastore types.

[ntoll]

If no default is given for a datastore key/value pair, depending on the type of the value, a sensible default value should be used.

Suggested first defaults:

• str = ""
• int = 0
• float = 0.0
• bool = False
• list = []
• dict = {}

TL;DR, we don't want null/None values.

This needs to be reflected in the default values for widget properties too.