NotNullConstraintValidator doesn't catch empty strings, only list/complex data types

Problem/Motivation

I applied NotNull to a string value, since we do not have a NotEmptyString constraint. This constraint validator states "Overrides the symfony validator to handle empty Typed Data structures."

    $typed_data = $this->getTypedData();
    if (($typed_data instanceof ListInterface || $typed_data instanceof ComplexDataInterface) && $typed_data->isEmpty()) {
      $value = NULL;
    }

However it only calls isEmpty if the typed data object is a list/sequence or complex/mapping data type.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet