Support top-level run_udf in validation mode (like in wet run) #471

#364 added a validation error item from top-level run_udf usage:

which was ok at the time as the python client only shows this as a warning, but still allows user to submit the job.

Since Open-EO/openeo-web-editor#309 (comment) however, the web editor started blocking workflows when pre-flight validation returns errors.

This makes it impossible to submit jobs involving top-level run_udf nodes through web editor