Removes default parameter to fix dealer reg

For some reason, defining the default badge type broke dealer registration. This replaces that with a check to see if the badge type is set.
Ah, this is something kind of dumb about how I'm doing these global config variables.  Next year the thing we're going to do is have a single global variable ``c`` so that instead of saying ``ATTENDEE_BADGE`` we can say ``c.ATTENDEE_BADGE`` which will make this kind of problem go away.

If you're curious about why this would be an issue, here's the deal.  In Python when you say ``from some_module import *`` then you're grabbing all of the global variables from that module so that you have a reference to them in the current module.  Fair enough, but suppose you have the following case.

```python
# filename: a.py
x = 5
from b import *
y = 6
f()
```

and then also have

```python
# filename: b.py
from a import *
def f():
    print(x + y)
```

Ok, so that should work, right?  Actually no - because at the time that ``b`` is imported, the ``a`` module only has ``x`` defined and ``y`` doesn't get defined until after b is imported.  So we end up in a situation where saying ``from a import *`` may or may not import all of the global variables in A depending on when in the import process this takes place.

In general, the Python community discourages the use of ``from some_module import *`` for this reason; the recommended approach is to explicitly enumerate everything that you're importing.  (It's okay to say ``from some_module import *`` if you define a ``__all__`` in the module which explicitly enumerates what ``*`` will mean in the import.)

There's an open ticket for this refactor, but I decided not to tackle doing that this year since I was already doing a bunch of config refactoring.

So anyway, the reason why you can say ``ATTENDEE_BADGE`` within a function but not at the global level is because we do some shenanigans in ``common.py`` that basically jam all global variables into every module even if they weren't defined yet when the module was first imported.  Which is REALLY NOT OKAY for me to have done, but this is kind of an artifact of Uber being a >7 year old codebase which I've been gradually cleaning up as I've gotten more familiar with good Python practices :)