Enable nullable reference types in runners

src/Abc.Zebus.Directory.Runner/Abc.Zebus.Directory.Runner.csproj

@@ -5,6 +5,7 @@
     <OutputType>Exe</OutputType>
     <Version>$(ZebusDirectoryVersion)</Version>
     <IsPackable>false</IsPackable>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>

src/Abc.Zebus.Persistence.Runner/Abc.Zebus.Persistence.Runner.csproj

@@ -5,6 +5,7 @@
     <OutputType>Exe</OutputType>
     <Version>$(ZebusPersistenceVersion)</Version>
     <IsPackable>false</IsPackable>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>

src/Abc.Zebus.Persistence.Runner/Program.cs

@@ -54,7 +54,7 @@ namespace Abc.Zebus.Persistence.Runner
                 var inMemoryMessageMatcherInitializer = busFactory.Container.GetInstance<InMemoryMessageMatcherInitializer>();
                 inMemoryMessageMatcherInitializer.BeforeStart();
 
-                OldestNonAckedMessageUpdaterPeriodicAction oldestNonAckedMessageUpdaterPeriodicAction = null;
+                OldestNonAckedMessageUpdaterPeriodicAction? oldestNonAckedMessageUpdaterPeriodicAction = null;
                 if (useCassandraStorage)
                 {
                     oldestNonAckedMessageUpdaterPeriodicAction = busFactory.Container.GetInstance<OldestNonAckedMessageUpdaterPeriodicAction>();