gitea-branch-cleanup: drop redundant nil guards on PR model fields

pr.Head/pr.Base of a PR returned by GetPullRequestsWithState are never
nil in real Gitea data. Drop the "!= nil" part of the branch-collection
guards, keeping the meaningful "Ref != ''" business condition. Complete
the test mock accordingly.

Assisted-by: Anthropic:claude-opus-4.8

utils/gitea-branch-cleanup/cleanup.go

@@ -58,10 +58,10 @@ func (s *CleanupService) IdentifyCandidates(ctx context.Context, opts CleanupOpt
 		}
 
 		for _, pr := range openPRs {
-			if pr.Head != nil && pr.Head.Ref != "" {
+			if pr.Head.Ref != "" {
 				openPRBranches[pr.Head.Ref] = true
 			}
-			if pr.Base != nil && pr.Base.Ref != "" {
+			if pr.Base.Ref != "" {
 				openPRBranches[pr.Base.Ref] = true
 			}
 		}

utils/gitea-branch-cleanup/cleanup_test.go

@@ -31,6 +31,7 @@ func TestCleanupService_IdentifyCandidates(t *testing.T) {
 	mockGitea.EXPECT().GetPullRequestsWithState(owner, repo, "open").Return([]*models.PullRequest{
 		{
 			Head: &models.PRBranchInfo{Ref: "temp-has-pr"},
+			Base: &models.PRBranchInfo{Ref: "main"},
 		},
 	}, nil)
 
@@ -121,6 +122,7 @@ func TestCleanupService_IdentifyCandidates_MultiplePrefixes(t *testing.T) {
 	mockGitea.EXPECT().GetPullRequestsWithState(owner, repo, "open").Return([]*models.PullRequest{
 		{
 			Head: &models.PRBranchInfo{Ref: "test-has-pr"},
+			Base: &models.PRBranchInfo{Ref: "main"},
 		},
 	}, nil)