Allow empty enum with non-empty mixins

[AndrewFossAWS]

Issue #, if available:
Customer tries to apply a non-empty enum mixin to an empty enum:

@mixin
enum EnumMixin {
    VALUE = "value"
}

enum EmptyEnum with [EnumMixin] {}

Smithy build fails with enum must have at least one entry. This was caused by mixin enum members not getting populated to the synthetic enum trait.

Description of changes:
Allow empty enum with non-empty mixins

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