allow before/after diff to be NoneType (#62582)

when creating or deleting an object (e.g. via an API), before/after can
be `None` (or at least represented as such by the used library). to
avoid modules havig to do

    diff={'before': before or '', 'after': after or ''}

let's just convert `None` to an empty string that can be diffed properly

1 files changed, 28 insertions, 0 deletions

diff --git a/test/units/plugins/callback/test_callback.py b/test/units/plugins/callback/test_callback.py
index 52233800f3..906f00040c 100644
--- a/test/units/plugins/callback/test_callback.py
+++ b/test/units/plugins/callback/test_callback.py
@@ -365,6 +365,34 @@ class TestCallbackDiff(unittest.TestCase):
 
             '''))
 
+    def test_diff_before_none(self):
+        self.assertMultiLineEqual(
+            self._strip_color(self.cb._get_diff({
+                'before': None,
+                'after': 'one line\n',
+            })),
+            textwrap.dedent('''\
+                --- before
+                +++ after
+                @@ -0,0 +1 @@
+                +one line
+
+            '''))
+
+    def test_diff_after_none(self):
+        self.assertMultiLineEqual(
+            self._strip_color(self.cb._get_diff({
+                'before': 'one line\n',
+                'after': None,
+            })),
+            textwrap.dedent('''\
+                --- before
+                +++ after
+                @@ -1 +0,0 @@
+                -one line
+
+            '''))
+
 
 class TestCallbackOnMethods(unittest.TestCase):
     def _find_on_methods(self, callback):