Make sure generated references metadata isn't overriden by static metadata

[whd]

https://github.com/mozilla/bigquery-etl/pull/4613/files#r1408290814

I could be misinterpreting what's happening here but it seems like specifying references: {} explicitly in main branch metadata may sometimes cause references to not be correctly generated/overridden in generated metadata. I'm not aware of any case where we'd want an empty static value for references to be preferred to a generated value, but there may be complex generated cases where some non-empty value should be preferred.

Another option might be to in CI enforce not setting references at all and require it be generated.

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