Through REST v2 (and likely the PAPI aswell), creating a ContentType with an Image FieldType, and omitting the validator configurations on it (since they are optional, as seen here) will cause an error:

Warning: Invalid argument supplied for foreach() 
in .../ezp5/vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/FieldType/Image/Type.php
line 228

Looking at the code, I tracked the error through the following files:

1. Core/Repository/ContentTypeService:581; in a situation where the configurations are not passed through the REST request, $fieldDefinitionCreateStruct->validatorConfiguration is NULL.

2. Core/FieldType/Image/Type.php:224 attempts to iterate through this. Since the argument is NULL, an error is thrown.

This issue may be present in other places. An example of a place where it WAS considered is Core/FieldType/TextLine/Type:45, where the argument is cast to an array in the iteration.