Enum values are incorrectly marked as deprecated

[symm]

Steps to reproduce:

1. Create a schema containing an enum using AbstractEnumType
2. Do an introspection query using e.g. get-graphql-schema
3. The enum values are incorrectly reported as @deprecated because the deprecationReason defaults to an empty string instead of null

[viniychuk]

@symm thanks for reporting, will take care of it.