Fix the issue with empty source_models for (multi-)selects

[loekvangool]

Description (*)

My install has many multiselect/select attributes without a source_model in the eav_attribute table. The issue is described here: https://magento.stackexchange.com/a/59311/73868. It has lead to issues while trying to set attribute values from a script:

PHP Fatal error:  Uncaught Mage_Eav_Exception: Source model "" not found for attribute "<code>" in app/Mage.php:634

Expected behavior (*)

It should be impossible to create a multiselect attribute without setting a source model

Benefits

Less issues with unset source models.

Additional information

This is also for discussion/tracking, still some work to verify the issue and fix.