THRIFT-5909: Addressed discrepancy in the non-accelerated union implementation

[kpumuk]

When binary extension is not available (for example, in JRuby), Thrift::Union throws an unexpected NoMethodError instead of the intended error:

  1) Union Thrift::Union should raise for wrong set field when hash initialized and type checking is off
     Failure/Error: expect { Thrift::Serializer.new.serialize(union) }.to raise_error(RuntimeError, "set_field is not valid for this union!")

       expected RuntimeError with "set_field is not valid for this union!", got #<NoMethodError: undefined method '[]' for nil> with backtrace:
         # ./lib/thrift/union.rb:78:in 'write'
         # ./lib/thrift/serializer/serializer.rb:29:in 'serialize'
         # ./spec/union_spec.rb:54:in 'block in <main>'
         # ./spec/union_spec.rb:54:in 'block in <main>'
     # ./spec/union_spec.rb:54:in 'block in <main>'  

Corresponding extension code: https://github.com/apache/thrift/blob/master/lib/rb/ext/struct.c#L673-L675

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