improvement: make mutation arguments non-null

[rbino]

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.

Contributor checklist

• [x] Features include unit/acceptance tests

[zachdaniel]

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