Fix Query variables storage after max errors

[ydah]

Fix GraphQL::Query::Variables so @storage remains a hash when variable validation stops after reaching validate_max_errors.

This is a narrow edge case: normal execution already returns validation errors, but callers may still inspect result.query.variables for logging, instrumentation, or error handling. Previously, the break inside each_with_object could make the whole expression return nil, causing delegated methods like length, to_h, or [] to raise NoMethodError.

[rmosolgo]

Thanks for taking care of this!