Fix Bug in Issue Config when only contact links are set (#26521)

Blank Issues should be enabled if they are not explicit disabled through
the `blank_issues_enabled` field of the Issue Config. The Implementation
has currently a Bug: If you create a Issue Config file with only
`contact_links` and without a `blank_issues_enabled` field,
`blank_issues_enabled` is set to false by default.

The fix is only one line, but I decided to also improve the tests to
make sure there are no other problems with the Implementation.

This is a bugfix, so it should be backported to 1.20.

tests/integration/api_repo_file_helpers.go

@@ -6,6 +6,7 @@ package integration
 import (
 	"strings"
 
+	"code.gitea.io/gitea/models"
 	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/git"
@@ -29,6 +30,32 @@ func createFileInBranch(user *user_model.User, repo *repo_model.Repository, tree
 	return files_service.ChangeRepoFiles(git.DefaultContext, repo, user, opts)
 }
 
+func deleteFileInBranch(user *user_model.User, repo *repo_model.Repository, treePath, branchName string) (*api.FilesResponse, error) {
+	opts := &files_service.ChangeRepoFilesOptions{
+		Files: []*files_service.ChangeRepoFile{
+			{
+				Operation: "delete",
+				TreePath:  treePath,
+			},
+		},
+		OldBranch: branchName,
+		Author:    nil,
+		Committer: nil,
+	}
+	return files_service.ChangeRepoFiles(git.DefaultContext, repo, user, opts)
+}
+
+func createOrReplaceFileInBranch(user *user_model.User, repo *repo_model.Repository, treePath, branchName, content string) error {
+	_, err := deleteFileInBranch(user, repo, treePath, branchName)
+
+	if err != nil && !models.IsErrRepoFileDoesNotExist(err) {
+		return err
+	}
+
+	_, err = createFileInBranch(user, repo, treePath, branchName, content)
+	return err
+}
+
 func createFile(user *user_model.User, repo *repo_model.Repository, treePath string) (*api.FilesResponse, error) {
 	return createFileInBranch(user, repo, treePath, repo.DefaultBranch, "This is a NEW file")
 }