[vm] Relax assertion for empty record type

TEST=co19/LanguageFeatures/Records/upper_and_lower_bounds_A04_t01
(Tested manualy by applying 4b6a8f3 and running
"tools/test.py -n dartk-strong-linux-debug-x64 co19/LanguageFeatures/Records/upper_and_lower_bounds_A04_t01").

Issue: #49719
Change-Id: Ib5658ac776a75913e617095f8861baba25f13de3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/261800
Reviewed-by: Ryan Macnak <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>
Auto-Submit: Alexander Markov <[email protected]>

Author: alexmarkov

runtime/vm/object.cc

@@ -27111,7 +27111,7 @@ void RecordType::SetFieldTypeAt(intptr_t index,
 }
 
 void RecordType::set_field_types(const Array& value) const {
-  ASSERT(!value.IsNull() && (value.Length() > 0));
+  ASSERT(!value.IsNull());
   untag()->set_field_types(value.ptr());
 }