Don't check for dynamic label if there is no label

[lognaturel]

Closes #532

Why is this the best possible solution? Were any other approaches considered?

This is a quick prevention of the crash. I think we should also consider #534.

I also added a couple of tests around numeric choice names because I initially thought that might be the issue and it does seem like something that could regress.

What are the regression risks?

It's an added None check only so I don't believe there are any.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests_v1
• run nosetests and verified all tests pass
• run black pyxform to format code
• verified that any code or assets from external sources are properly credited in comments

[lognaturel]

@gushil would you be up for a review? This is a fix for #521 which assumed labels were required.

[gushil]

I'm Ok with these changes.
No test for dynamic labels here because they're already tested in, for example, #521 right?

[lognaturel]

No test for dynamic labels here because they're already tested in, for example, #521 right?

That's right.