[api] Fix compilation error for `UNIMPLEMENTED()` method

Return `nullptr` from `ArrayBuffer::Allocator::Reserve` because
apparently not doing so results in compile errors for some people.

BUG=

Ref: https://github.com/nodejs/node/issues/13392
Review-Url: https://codereview.chromium.org/2929993003
Cr-Commit-Position: refs/heads/master@{#45886}

src/api.cc

@@ -453,7 +453,10 @@ void V8::SetSnapshotDataBlob(StartupData* snapshot_blob) {
   i::V8::SetSnapshotBlob(snapshot_blob);
 }
 
-void* v8::ArrayBuffer::Allocator::Reserve(size_t length) { UNIMPLEMENTED(); }
+void* v8::ArrayBuffer::Allocator::Reserve(size_t length) {
+  UNIMPLEMENTED();
+  return nullptr;
+}
 
 void v8::ArrayBuffer::Allocator::Free(void* data, size_t length,
                                       AllocationMode mode) {