ref(grouping): Simplify handling of variant name and exception data

[lobsterkatie]

During grouping, we pass data from one component strategy to another by storing it in the grouping context. In that context, two values - the variant name and a blob of exception data - are only sometimes defined. Until recently, the GroupingContext class supported key-lookup using subscript notation (context[some_key]) the same way a dictionary would, but lacked the safer .get() method. We therefore currently initialize both values to None, to prevent raising a KeyError if the values haven't been set.

Now that .get() has been implemented, though, we don't need to do that. This PR therefore switches the way we access them to use .get(), and removes the default initialization. In order to enforce when variant_name is or isn't expected to be defined, it also extends the practice already followed in some strategies of asserting on its existence (or lack thereof). This ensures that if we ever make changes to the grouping code, we'll know right away if our assumptions have been violated.