Guard against null in exportedDeclarations.

I was not able to find the root reason of the problem :-(

R=brianwilkerson@google.com

Bug: dart-lang/sdk#38261
Change-Id: Ib9a78366548688eab231fc941b29f608bdb6b15e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/116261
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/services/available_declarations.dart

@@ -590,7 +590,7 @@ class DeclarationsTracker {
         file.path,
         file.uri,
         file.isLibraryDeprecated,
-        file.exportedDeclarations,
+        file.exportedDeclarations ?? const [],
       );
       _idToLibrary[file.id] = library;
       _changesController.add(
@@ -735,7 +735,7 @@ class DeclarationsTracker {
           libraryFile.path,
           libraryFile.uri,
           libraryFile.isLibraryDeprecated,
-          libraryFile.exportedDeclarations,
+          libraryFile.exportedDeclarations ?? const [],
         );
         _idToLibrary[library.id] = library;
         changedLibraries.add(library);