fix(c-api) Allow finalizer to be NULL.

lib/c-api/src/wasm_c_api/externals/function.rs

@@ -7,6 +7,7 @@ use std::ffi::c_void;
 use std::sync::Arc;
 use wasmer::{Function, Instance, RuntimeError, Val};
 
+#[derive(Debug)]
 #[allow(non_camel_case_types)]
 pub struct wasm_func_t {
     pub(crate) inner: Function,
@@ -91,12 +92,11 @@ pub unsafe extern "C" fn wasm_func_new_with_env(
     function_type: Option<&wasm_functype_t>,
     callback: Option<wasm_func_callback_with_env_t>,
     env: *mut c_void,
-    finalizer: Option<wasm_env_finalizer_t>,
+    finalizer: wasm_env_finalizer_t,
 ) -> Option<Box<wasm_func_t>> {
     let store = store?;
     let function_type = function_type?;
     let callback = callback?;
-    let finalizer = finalizer?;
 
     let func_sig = &function_type.inner().function_type;
     let num_rets = func_sig.results().len();