[vm] Return empty array for constant table rather than null when there are no libraries.

This is follow-up to https://dart-review.googlesource.com/c/sdk/+/87613.

Change-Id: I4c26b07c36b1db31a20c11c1194f66f9ce56a03f
Reviewed-on: https://dart-review.googlesource.com/c/88201
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>
diff --git a/runtime/vm/kernel_loader.cc b/runtime/vm/kernel_loader.cc
index dd5d0d1..43a4dbe 100644
--- a/runtime/vm/kernel_loader.cc
+++ b/runtime/vm/kernel_loader.cc
@@ -391,7 +391,7 @@
 
 const Array& KernelLoader::ReadConstantTable() {
   if (program_->library_count() == 0) {
-    return Array::null_array();
+    return Array::empty_array();
   }
   // We use the very first library's toplevel class as an owner for an
   // [ActiveClassScope]