set function string when creating generic param model parameterizer

Summary: While working with the generic data fitting and fit inspector view, I noticed crashes when trying to inspect the fit result of a generic parameter model. Selecting the image node in the DataManager caused a crash. As it turns out, the parameterizer for a generic param model wasn't given the function when created, which causes the formula parser to try and parse an empty string.

Test Plan: Make a generic parameter model fit and inspect it. There should be no crashes, and results should be shown as expected.

Reviewers: O3 MITK Reviewer Group II, kompan, floca, kislinsk

Reviewed By: O3 MITK Reviewer Group II, floca, kislinsk

Differential Revision: https://phabricator.mitk.org/D876