JsonSchema validation regression with the RequiredValidation ext impl

This commit comes with a regression in the JsonSchema validation https://github.com/Talend/component-runtime/commit/f2275141273e82a13ea122b3838c1bf17bc35cd1

Before the change, and by using the RequiredValidation from johnzon, if a required parameter is missing, an error is raised thanks to that "isNull" line of code (https://github.com/apache/johnzon/blob/master/johnzon-jsonschema/src/main/java/org/apache/johnzon/jsonschema/spi/builtin/RequiredValidation.java#L61). With our own implementation, we filter out the 'null' element from the required list, by using this 'isPresent' method (https://github.com/Talend/component-runtime/blob/master/component-form/component-form-core/src/main/java/org/talend/sdk/component/form/internal/validation/spi/ext/RequiredValidation.java#L64).

With the Johnzon implementation, a @Required element which were missing returns a ValidationError. With our own implementation, it's not longer the case.