Make PrepareTestData default to return no branches

Tests started throwing null reference exceptions when we stopped using
an interface for BranchModel. This makes PrepareTestData default to
return no branches.

src/GitHub.App/ViewModels/GitHubPane/PullRequestCreationViewModel.cs

@@ -82,8 +82,9 @@ public PullRequestCreationViewModel(
                 .Where(_ => TargetBranch != null)
                 .Subscribe(x =>
                 {
-                    // HACK: Why is `t` null?
-                    if (!x.Any(t => t != null && t.Equals(TargetBranch)))
+                    //// HACK: Why is `t` null?
+                    //if (!x.Any(t => t != null && t.Equals(TargetBranch)))
+                    if (!x.Any(t => t.Equals(TargetBranch)))
                     {
                         TargetBranch = GitHubRepository.IsFork ? GitHubRepository.Parent.DefaultBranch : GitHubRepository.DefaultBranch;
                     }

test/GitHub.App.UnitTests/ViewModels/GitHubPane/PullRequestCreationViewModelTests.cs

@@ -101,6 +101,9 @@ static TestData PrepareTestData(
         api.GetRepository(Args.String, Args.String).Returns(Observable.Return(githubRepo));
         ms.ApiClient.Returns(api);
 
+        // Default to returning no branches
+        ms.GetBranches(null).ReturnsForAnyArgs(Observable.Empty<BranchModel>());
+
         // sets up the libgit2sharp repo and branch objects
         var l2repo = SetupLocalRepoMock(gitClient, gitService, remote, sourceBranchName, sourceBranchIsTracking);