Avoid 'NoneType' error when format conversion

Error raise when changing a 'None' data to a new format:
"TypeError: 'NoneType' object is not iterable"
we also met that bug before in the previous patch [1]

This change is to handle the 'None' data and avoid
the error

[1] https://review.openstack.org/#/c/420420/

Change-Id: I649b4fc65ef7c19b8193b07f3bd59f00e6095f9f
Closes-bug: #1656363

osc_lib/tests/test_utils.py

@@ -498,11 +498,13 @@ class TestFindResource(test_utils.TestCase):
                          utils.format_dict({'a': 'b', 'c': 'd', 'e': 'f'}))
         self.assertEqual(expected,
                          utils.format_dict({'e': 'f', 'c': 'd', 'a': 'b'}))
+        self.assertEqual(None, utils.format_dict(None))
 
     def test_format_list(self):
         expected = 'a, b, c'
         self.assertEqual(expected, utils.format_list(['a', 'b', 'c']))
         self.assertEqual(expected, utils.format_list(['c', 'b', 'a']))
+        self.assertEqual(None, utils.format_list(None))
 
     def test_format_list_of_dicts(self):
         expected = "a='b', c='d'\ne='f'"
@@ -512,6 +514,7 @@ class TestFindResource(test_utils.TestCase):
         self.assertEqual(expected, utils.format_list_of_dicts(unsorted_data))
         self.assertEqual('', utils.format_list_of_dicts([]))
         self.assertEqual('', utils.format_list_of_dicts([{}]))
+        self.assertEqual(None, utils.format_list_of_dicts(None))
 
     def test_format_list_separator(self):
         expected = 'a\nb\nc'

osc_lib/utils.py

@@ -192,6 +192,9 @@ def format_dict(data):
     :rtype: a string formatted to key='value'
     """
 
+    if data is None:
+        return None
+
     output = ""
     for s in sorted(data):
         output = output + s + "='" + six.text_type(data[s]) + "', "
@@ -205,6 +208,8 @@ def format_list(data, separator=', '):
     :param separator: the separator to use between strings (default: ', ')
     :rtype: a string formatted based on separator
     """
+    if data is None:
+        return None
 
     return separator.join(sorted(data))
 
@@ -215,6 +220,8 @@ def format_list_of_dicts(data):
     :param data: a list of dicts
     :rtype: a string formatted to key='value' with dicts separated by new line
     """
+    if data is None:
+        return None
 
     return '\n'.join(format_dict(i) for i in data)