fix excessive gleam.mjs generation

This patch fixes excessive generation by checking if `modules` is empty.

Two other possible solutions have been ruled out:

- Checking for file existence may result in not updating outdated files
- Comparing contents is slow

compiler-core/src/codegen.rs

@@ -186,7 +186,11 @@ impl<'a> JavaScript<'a> {
             }
             self.js_module(writer, module, &js_name)?
         }
-        self.write_prelude(writer)?;
+
+        if !modules.is_empty() {
+            self.write_prelude(writer)?;
+        }
+
         Ok(())
     }