Empty model definitions crash the source generator

[erri120]

public partial class Foo : IModelDefinition { }

Will crash the source generator, but should probably just create a diagnostic:

3>CSC: Warning CS8785 : Generator 'ModelGenerator' failed to generate source. It will not contribute to the output and compilation errors may occur as a result. Exception was of type 'ArgumentOutOfRangeException' with message 'Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')'.