Loosen DHCECK to allow for no HostImportModuleDynamically callback to be set

The DCHECK is firing because the fuzzer doesn't set any
HostImportModuleDynamically callback.  Previously
RunHostImportModuleDynamicallyCallback would not assert for this and
would just return a rejected promise.  After
https://chromium-review.googlesource.com/c/v8/v8/+/2620578, this results
in a failed DCHECK.

This change restores the old behavior by loosening the DCHECK such that
it only fails if both the deprecated and the new callback are set.

Bug: chromium:1172121
Change-Id: Ifda28eb28572a40d3752928997edf25d607b61c6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2659505Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Dan Clark <daniec@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#72462}

src/execution/isolate.cc

@@ -4157,9 +4157,9 @@ MaybeHandle<JSPromise> Isolate::RunHostImportModuleDynamicallyCallback(
     MaybeHandle<Object> maybe_import_assertions_argument) {
   v8::Local<v8::Context> api_context =
       v8::Utils::ToLocal(Handle<Context>(native_context()));
-  DCHECK_EQ(host_import_module_dynamically_callback_ != nullptr,
-            host_import_module_dynamically_with_import_assertions_callback_ ==
-                nullptr);
+  DCHECK(host_import_module_dynamically_callback_ == nullptr ||
+         host_import_module_dynamically_with_import_assertions_callback_ ==
+             nullptr);
 
   if (host_import_module_dynamically_callback_ == nullptr &&
       host_import_module_dynamically_with_import_assertions_callback_ ==