Fix interop tests

Currently the interop tests are failing:

/avro/lang/ruby/lib/avro/schema_validator.rb:127:in `validate_map': undefined method `keys' for nil:NilClass (NoMethodError)
from /avro/lang/ruby/lib/avro/schema_validator.rb:103:in `validate_recursive'
from /avro/lang/ruby/lib/avro/schema_validator.rb:156:in `block in validate_possible_types'
from /avro/lang/ruby/lib/avro/schema_validator.rb:154:in `map'
from /avro/lang/ruby/lib/avro/schema_validator.rb:154:in `validate_possible_types'
from /avro/lang/ruby/lib/avro/schema_validator.rb:141:in `validate_union'
from /avro/lang/ruby/lib/avro/schema_validator.rb:105:in `validate_recursive'
from /avro/lang/ruby/lib/avro/schema_validator.rb:110:in `block in validate_recursive'
from /avro/lang/ruby/lib/avro/schema_validator.rb:108:in `each'
from /avro/lang/ruby/lib/avro/schema_validator.rb:108:in `validate_recursive'
from /avro/lang/ruby/lib/avro/schema_validator.rb:67:in `validate!'
from /avro/lang/ruby/lib/avro/schema.rb:97:in `validate'
from /avro/lang/ruby/lib/avro/io.rb:503:in `write_data'
from /avro/lang/ruby/lib/avro/io.rb:499:in `write'
from /avro/lang/ruby/lib/avro/ipc.rb:136:in `write_handshake_request'
from /avro/lang/ruby/lib/avro/ipc.rb:105:in `request'
from lang/ruby/test/tool.rb:69:in `send_message'
from lang/ruby/test/tool.rb:137:in `main'
from lang/ruby/test/tool.rb:143:in `<main>'