Initialization / Validation of required non-existent data attributes

[sdirix]

When an attribute is set to required in JSONSchema and the currently bound data does not contain said attribute a validation error is shown XXX is a required property.

When for example an attribute of type string is required and has no other constraints, then entering and removing a value in the corresponding input will remove the validation error. This is the case because now the empty string "" is bound in the data. However for the user the form does not look different than before.

This situation could be improved for example by initializing the data attributes or changing the initial validation. The solution should be configurable and be reasonable for the default use case.

[edgarmueller]

Duplicate of #1168