Don't generate if an enum has no cases yet

[jameshulse]

We happen to have a placeholder backed enum in our code base with no cases yet.

The current output in our generated.d.ts when 'transform_to_native_enums' => false looks like:

declare namespace ... {
export type Placeholder = ; // Syntax error as nothing between '=' and ';'
}

This is a syntax error and might be causing type inference issues in the rest of our application.