[formatter] allow accessing 'util.NONE' via global '_nul'

docs/formatting.md

@@ -272,6 +272,12 @@ Replacement field names that are available in all format strings.
     <td><code>{_now:%Y-%m}</code></td>
     <td><code>2022-08</code></td>
 </tr>
+<tr>
+    <td><code>_nul</code></td>
+    <td>Universal <code>null</code> value</td>
+    <td><code>{date|_nul:%Y-%m}</code></td>
+    <td><code>None</code></td>
+</tr>
 <tr>
     <td rowspan="2"><code>_lit</code></td>
     <td rowspan="2">String literals</td>

gallery_dl/formatter.py

@@ -476,6 +476,7 @@ _GLOBALS = {
     "_env": lambda: os.environ,
     "_lit": lambda: _literal,
     "_now": datetime.datetime.now,
+    "_nul": lambda: util.NONE,
 }
 _CONVERSIONS = {
     "l": str.lower,

test/test_formatter.py

@@ -351,6 +351,14 @@ class TestFormatter(unittest.TestCase):
         self.assertRegex(out1, r"^\d{4}-\d\d-\d\d \d\d:\d\d:\d\d(\.\d+)?$")
         self.assertNotEqual(out1, out2)
 
+    def test_globals_nul(self):
+        value = "None"
+
+        self._run_test("{_nul}"         , value)
+        self._run_test("{_nul[key]}"    , value)
+        self._run_test("{z|_nul}"       , value)
+        self._run_test("{z|_nul:%Y%m%s}", value)
+
     def test_literals(self):
         value = "foo"