Require widget to be configured for values loaded as null

[sweh]

Originally reported by: Thomas Lotze (Bitbucket: tlotze, GitHub: tlotze)

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Widget configuration is optional and falls back to value-type sniffing. This will obviously break with fields that may have a null value. When loading values, it should be considered an error for a value to be null and at the same time have no widget configured.

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• Bitbucket: https://bitbucket.org/gocept/gocept.jsform/issue/8