lib/ignore: Don't create empty ".stignore" files (fixes #6190) (#6197)

This will:

1. prevent creation of a new .stignore if there are no ignore patterns
2. delete an existing .stignore if all ignore patterns are removed

lib/ignore/ignore.go

@@ -545,6 +545,14 @@ func parseIgnoreFile(fs fs.Filesystem, fd io.Reader, currentFile string, cd Chan
 
 // WriteIgnores is a convenience function to avoid code duplication
 func WriteIgnores(filesystem fs.Filesystem, path string, content []string) error {
+	if len(content) == 0 {
+		err := filesystem.Remove(path)
+		if fs.IsNotExist(err) {
+			return nil
+		}
+		return err
+	}
+
 	fd, err := osutil.CreateAtomicFilesystem(filesystem, path)
 	if err != nil {
 		return err