Ensure called default_sort_func is not None

It would be easier to just default to `sorted` instead of `None`, but since `None` is an option, we have to test for it anyway.

Signed-off-by: Michael Tiemann <[email protected]>

Author: MichaelTiemannOSC

pint/delegates/formatter/html.py

@@ -78,6 +78,12 @@ def format_unit(
         self, unit: PlainUnit, uspec: str = "", **babel_kwds: Unpack[BabelKwds]
     ) -> str:
         units = format_compound_unit(unit, uspec, **babel_kwds)
+        if unit._REGISTRY.formatter.default_sort_func is not None:
+            sort_func = lambda x: unit._REGISTRY.formatter.default_sort_func(
+                x, unit._REGISTRY
+            )
+        else:
+            sort_func = None
 
         return formatter(
             units,
@@ -87,9 +93,7 @@ def format_unit(
             division_fmt=r"{}/{}",
             power_fmt=r"{}<sup>{}</sup>",
             parentheses_fmt=r"({})",
-            sort_func=lambda x: unit._REGISTRY.formatter.default_sort_func(
-                x, unit._REGISTRY
-            ),
+            sort_func=sort_func,
         )
 
     def format_quantity(

pint/delegates/formatter/plain.py

@@ -259,6 +259,12 @@ def format_unit(
         self, unit: PlainUnit, uspec: str = "", **babel_kwds: Unpack[BabelKwds]
     ) -> str:
         units = format_compound_unit(unit, uspec, **babel_kwds)
+        if unit._REGISTRY.formatter.default_sort_func is not None:
+            sort_func = lambda x: unit._REGISTRY.formatter.default_sort_func(
+                x, unit._REGISTRY
+            )
+        else:
+            sort_func = None
 
         return formatter(
             units,
@@ -269,9 +275,7 @@ def format_unit(
             power_fmt="{}{}",
             parentheses_fmt="({})",
             exp_call=pretty_fmt_exponent,
-            sort_func=lambda x: unit._REGISTRY.formatter.default_sort_func(
-                x, unit._REGISTRY
-            ),
+            sort_func=sort_func,
         )
 
     def format_quantity(