Reset Region field for client and server Geo annotations

[stephen-soltesz]

This change clears the Region field for the annotation datatype.

This change addresses https://github.com/m-lab/etl/issues/1069 for the annotation datatype (which includes annotations generated by the synthetic uuid annotation export process (which have the Region field set) as well as annotations collected natively by the uuid-annotator (which do not).

Because the annotation Geo fields include the JSON tag omitempty, the v2 data pipeline will load these values as NULL in BigQuery.

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 7284

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 65.243%

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Totals
Change from base Build 7278:	0.02%
Covered Lines:	3972
Relevant Lines:	6088

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💛 - Coveralls