Infer Null for return type of functions with empty returns.

pkg/analyzer/lib/src/summary/link.dart

Index: pkg/analyzer/lib/src/summary/link.dart
diff --git a/pkg/analyzer/lib/src/summary/link.dart b/pkg/analyzer/lib/src/summary/link.dart
index c1f9f67b553335322ec528030db7408442043325..a97a11b7a794dfdc5aa788989eb7db2b5c5509f8 100644
--- a/pkg/analyzer/lib/src/summary/link.dart
+++ b/pkg/analyzer/lib/src/summary/link.dart
@@ -4729,6 +4729,7 @@ class TypeProviderForLink extends TypeProviderBase {
   InterfaceType _functionType;
   InterfaceType _futureDynamicType;
   InterfaceType _futureNullType;
+  InterfaceType _futureOrNullType;
   InterfaceType _futureOrType;
   InterfaceType _futureType;
   InterfaceType _intType;
@@ -4779,6 +4780,10 @@ class TypeProviderForLink extends TypeProviderBase {
       _futureNullType ??= futureType.instantiate(<DartType>[nullType]);
 
   @override
+  InterfaceType get futureOrNullType =>
+      _futureOrNullType ??= futureOrType.instantiate(<DartType>[nullType]);
+
+  @override
   InterfaceType get futureOrType =>
       _futureOrType ??= _buildInterfaceType(_linker.asyncLibrary, 'FutureOr');