derive-typescript: fix codegen for empty enums

We saw a derivation that failed validation because the generated Typescript
code was syntactically invalid. This was caused by the inferred `Shape` having
an empty set of enum variants (`enum_: Some([])`).  The empty enum variants
cause the generated code to omit any sort of type, which causes Deno to fail to
parse it.

I'm not completely certain whether the fault is with the Typescript code
generation or the Shape inferrence, but it's pretty easy to filter out empty
enums in the code generation, which results in generated code correctly having
the `never` type.

I also added a debug log of the generated types file, so that this type of
thing is easier to debug in the future.

crates/derive-typescript/src/codegen/mapper.rs

@@ -115,7 +115,7 @@ impl Mapper {
             return AST::Unknown;
         }
         // Is this an enum? Just emit the variants.
-        if let Some(enum_) = &shape.enum_ {
+        if let Some(enum_) = shape.enum_.as_ref().filter(|e| !e.is_empty()) {
             return AST::Union {
                 variants: enum_
                     .iter()

crates/derive-typescript/src/lib.rs

@@ -178,6 +178,7 @@ fn validate(validate: derive::request::Validate) -> anyhow::Result<derive::respo
 
     let types_url = format!("{project_root}/{GENERATED_PREFIX}/{}.ts", collection.name);
     let types_content = codegen::types_ts(&collection, &transforms);
+    tracing::debug!(%types_content, "generated TS types");
 
     let mut generated_files: Vec<(String, String)> = vec![
         (types_url.clone(), types_content.clone()),