Fix download of votables for ucd fields holding a None value

[kimakan]

See #148

[coveralls]

Pull Request Test Coverage Report for Build 4144451062

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 64.377%

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Totals
Change from base Build 4104281142:	0.0%
Covered Lines:	4686
Relevant Lines:	7279

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💛 - Coveralls