types.EnumMeta: handle Null value

[kgadek]

Ref: profusion/sgqlc#213

[coveralls]

Pull Request Test Coverage Report for Build 2887599719

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 2186990982:	0.0%
Covered Lines:	1553
Relevant Lines:	1553

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💛 - Coveralls

[barbieri]

thank you :-)