Fix bug when exclude pattern is None

.gitignore

@@ -1,2 +1,3 @@
 .idea/
+__pycache__/
 test/

simple_backup/simple_backup.py

@@ -185,9 +185,10 @@ class Backup:
                     fp.write('\n')
 
         with open(self._exclude_path, 'w') as fp:
-            for e in self.exclude:
-                fp.write(e)
-                fp.write('\n')
+            if self.exclude is not None:
+                for e in self.exclude:
+                    fp.write(e)
+                    fp.write('\n')
 
         if self.keep != -1 and self.remove_before:
             self.remove_old_backups()