Do not include docstring if empty

Closes #3945

Author: kflansburg

crates/backend/src/codegen.rs

@@ -867,10 +867,18 @@ impl ToTokens for ast::ImportType {
 
         let no_deref = self.no_deref;
 
+        let doc = if doc_comment.is_empty() {
+            quote! {}
+        } else {
+            quote! {
+                #[doc = #doc_comment]
+            }
+        };
+
         (quote! {
             #[automatically_derived]
             #(#attrs)*
-            #[doc = #doc_comment]
+            #doc
             #[repr(transparent)]
             #vis struct #rust_name {
                 obj: #internal_obj