I18n: restore unit messages after c++ migration broke extraction

The units defined in blenkernel/intern/unit.c were extracted using a
regex which contained `NULL`. Commit 129f78eee7 converted this file to
c++, and these `NULL` were replaced with `nullptr`, breaking the
regex.

This commit changes the regex to `nullptr` as well to restore the
translations.

Pull Request: https://projects.blender.org/blender/blender/pulls/110420

scripts/modules/bl_i18n_utils/settings.py

@@ -282,8 +282,8 @@ PYGETTEXT_KEYWORDS = (() +
     # NOTE: regex is a bit more complex than it would need too. Since the actual
     # identifier (`B_UNIT_DEF_`) is at the end, if it's simpler/too general it
     # becomes extremely slow to process some (unrelated) source files.
-    ((r"\{(?:(?:\s*\"[^\",]+\"\s*,)|(?:\s*\"\\\"\",)|(?:\s*NULL\s*,)){4}\s*" +
-      _msg_re + r"\s*,(?:(?:\s*\"[^\"',]+\"\s*,)|(?:\s*NULL\s*,))(?:[^,]+,){2}"
+    ((r"\{(?:(?:\s*\"[^\",]+\"\s*,)|(?:\s*\"\\\"\",)|(?:\s*nullptr\s*,)){4}\s*" +
+      _msg_re + r"\s*,(?:(?:\s*\"[^\"',]+\"\s*,)|(?:\s*nullptr\s*,))(?:[^,]+,){2}"
       + "(?:\|?\s*B_UNIT_DEF_[_A-Z]+\s*)+\}"),) +
 
     tuple((r"{}\(\s*" + _msg_re + r"\s*,\s*(?:" +