Prevent NoMethodError in group update creation

[bdragon]

We are seeing runtime exceptions due to calling a method on an argument that may be nil. This change properly handles nil arguments.

[jeffwidman]

Code change looks good, although I wonder how hard it'd be to add a unit test ensuring no future regressions?

[bdragon]

I wonder how hard it'd be to add a unit test ensuring no future regressions?

Dependabot::Updater::GroupUpdateCreation is a module and it looks like it doesn't have any tests. I'm guessing it wasn't really intended to be treated as a public API, the idea was probably that the functionality would be covered by more e2e-like updater tests or by tests for the classes it gets included in. It's included in three classes and one of those classes has tests, so I added a test there.