fix: Skip building FDv1 fallback factory list when no FDv1 synchronizer is configured

FDv2DataSystem.Create unconditionally constructed a one-entry list with
FactoryWithContext(clientContext)(dataSystemConfiguration.FDv1FallbackSynchronizer).
When FDv1FallbackSynchronizer was null this still produced a non-null
SourceFactory delegate that captured the null configurer. The OUTER
composite would then add a phantom FDv1 fallback entry; the moment an
action applier advanced to it the entry's factory invocation would
NullReferenceException inside FactoryWithContext.

Build the list as empty when no FDv1 fallback is configured, so the
OUTER composite simply has no FDv1 fallback entry and the data system
halts cleanly per the spec.

Author: keelerm84

pkgs/sdk/server/src/Internal/DataSystem/FDv2DataSystem.cs

@@ -79,14 +79,20 @@ public static FDv2DataSystem Create(Logger logger, Configuration configuration,
 
             var contextWithSelectorSource =
                 clientContext.WithSelectorSource(new SelectorSourceFacade(writeThroughStore));
+            // FDv1 fallback synchronizer is optional; only build a list entry when one is
+            // configured. An always-present list entry that captured a null configurer would
+            // throw NRE the moment the action applier advanced to the FDv1 fallback entry.
+            var fdv1FallbackFactories = dataSystemConfiguration.FDv1FallbackSynchronizer == null
+                ? new List<SourceFactory>()
+                : new List<SourceFactory>
+                {
+                    FactoryWithContext(clientContext)(dataSystemConfiguration.FDv1FallbackSynchronizer)
+                };
             var compositeDataSource = configuration.Offline ? Components.ExternalUpdatesOnly.Build(contextWithSelectorSource) : FDv2DataSource.CreateFDv2DataSource(
                 dataSourceUpdates,
                 dataSystemConfiguration.Initializers.Select(FactoryWithContext(contextWithSelectorSource)).ToList(),
                 dataSystemConfiguration.Synchronizers.Select(FactoryWithContext(contextWithSelectorSource)).ToList(),
-                new List<SourceFactory>
-                {
-                    FactoryWithContext(clientContext)(dataSystemConfiguration.FDv1FallbackSynchronizer)
-                },
+                fdv1FallbackFactories,
                 logger
             );