Fix definition of a "Required" property as defined in the schema.

[lawrence-mbf]

Currently, "required" properties error if the field is empty. However, the schema defines "required" properties as the existence of the field in the file itself.

MatNWB should instead use this method:
If the property is "required": The data should be written regardless of its contents.
If the property is "optional": The data should be written only if not empty.