* improvement: make mutation arguments non-null
As discussed in #105 and #110, put this behind an opt-in configuration to avoid
breaking existing code.
The ID in update mutations is always non-null if non-null mutation arguments are
allowed, while input is non-null if it's allowed _and_ there is at least a
non-null field in the input.
Document the newly added config variable in the getting started guide.
* chore: enable non-null mutation arguments in tests
