Fix null value for string array field crashes dataProvider in ra-data-graphql-simple

[stensrud]

typeof null === "object"

[djhi]

Thanks for contributing :) Can you please explain the issue solved by this PR?

[stensrud]

My GraphQL schema has fields with [String] type, the string being nullable. When a fetched object has a null-value, ex [null], the response parser will crash when trying to run Object.keys(null).reduce(...).

[djhi]

Thanks!