fix: mutation_root_name when there is no mutation type

[dylanahsmith]

@mikkoh please review

Problem

I get the following exception when there is no GraphQL mutation type:

NoMethodError: undefined method `fetch' for nil:NilClass
    /Users/dylansmith/src/graphql_schema/lib/graphql_schema.rb:18:in `mutation_root_name'
    /Users/dylansmith/src/graphql_schema/test/graphql_schema_test.rb:25:in `test_no_mutation_root'

because the default argument to fetch in mutation_root_name (i.e. @hash.fetch('mutationType', {})) wasn't being used since the mutationType key exists, it is just nil.

Solution

Use a condition to only call .fetch('name') if the mutation type is truthy.

[mikkoh]

LGTM 👍