Only generate JS null checks in debug mode

In non-debug mode Rust is already checking these pointers, so let's only
generate the relevant code in debug mode.

crates/backend/src/codegen.rs

@@ -985,7 +985,7 @@ impl ToTokens for ast::Enum {
                     _extra: &mut ::wasm_bindgen::convert::Stack,
                 ) -> Self {
                     #(#cast_clauses else)* {
-                        ::wasm_bindgen::throw("invalid enum value passed")
+                        ::wasm_bindgen::throw_str("invalid enum value passed")
                     }
                 }
             }