insights: create and update mutations have different nullability for filters

Created by: coury-clark

https://sourcegraph.com/search?q=context:%40sourcegraph/all+repo:%5Egithub%5C.com/sourcegraph/sourcegraph%24+file:%5Ecmd/frontend/graphqlbackend/insights%5C.graphql+viewControls&patternType=literal

The create line chart mutation has a nullable ViewControls object, but the update mutation has a non-nullable object.

We might want to just lump this in with the work to reimagine this entire model (try to reduce the complexity of chart types x series types in the GraphQL mutations)