Failure on emitting key that is not an array or GeoJSON

{markdown}
When something that is not an array or GeoJSON gets emitted as key it results in a failure:

{"error":"error","reason":"function_clause"}

This should be a proper error instead like when you emit an array that contains e.g. `null`:

{"error":"emit_key","reason":"The values of the key must be numbers or a GeoJSON geometry."}

This issue can be reproduced with this spatial function:

function (doc) { emit(null, null); }{markdown}