mutationのrequired: falseなargumentにdefault_valueをつける

[uplus]

省略した場合の引数を作らないと、unexcepted errorがでる

mutations/add_notice.rb:10:    argument :team_id,        ID,      required: false
mutations/apply_problem.rb:9:    argument :category_code,         String,                        required: false
mutations/apply_problem.rb:10:    argument :previous_problem_code, String,                        required: false
mutations/apply_problem.rb:13:    argument :open_at_begin,         Types::DateTime,               required: false
mutations/apply_problem.rb:14:    argument :open_at_end,           Types::DateTime,               required: false
mutations/apply_problem_environment.rb:9:    argument :team_number,  Integer, required: false
mutations/apply_score.rb:8:    argument :percent,   Integer, required: false
mutations/update_config.rb:7:    argument :key,   ID,     required: false
mutations/update_config.rb:8:    argument :value, String, required: false
types/query_type.rb:19:      argument :after, Types::DateTime, required: false