Allow file resolver provider to return null, just like package resolver provider

pkg/analysis_server/lib/src/analysis_server.dart

Index: pkg/analysis_server/lib/src/analysis_server.dart
diff --git a/pkg/analysis_server/lib/src/analysis_server.dart b/pkg/analysis_server/lib/src/analysis_server.dart
index 307ed65796f708da99f6bad321da7c2389253d63..72aff0bbe88d34d101fd7d4a0629b639085d0189 100644
--- a/pkg/analysis_server/lib/src/analysis_server.dart
+++ b/pkg/analysis_server/lib/src/analysis_server.dart
@@ -1713,11 +1713,11 @@ class ServerContextManagerCallbacks extends ContextManagerCallbacks {
     }
 
     resolvers.addAll(packageUriResolvers);
-    if (context.fileResolverProvider == null) {
-      resolvers.add(new ResourceUriResolver(resourceProvider));
-    } else {
-      resolvers.add(context.fileResolverProvider(folder));
+    UriResolver fileResolver;
+    if (context.fileResolverProvider != null) {
+      fileResolver = context.fileResolverProvider(folder);
     }
+    resolvers.add(fileResolver ?? new ResourceUriResolver(resourceProvider));
     return new SourceFactory(resolvers, disposition.packages);
   }
 }