[2.2.x] Fixed #31850 -- Fixed BasicExtractorTests.test_extraction_warning with xgettext 0.21+.

"format string with unnamed arguments cannot be properly localized"
warning is not raised in xgettext 0.21+.

This patch uses a message that causes an xgettext warning regardless of
the version.

Backport of 07a30f5 from master

Author: msmolens

AUTHORS

@@ -585,6 +585,7 @@ answer newbie questions, and generally made Django that much better:
     [email protected]
     Max Burstein <http://maxburstein.com>
     Max Derkachev <[email protected]>
+    Max Smolens <[email protected]>
     Maxime Lorant <[email protected]>
     Maxime Turcotte <[email protected]>
     Maximilian Merz <[email protected]>

tests/i18n/commands/code.sample

@@ -1,4 +1,4 @@
 from django.utils.translation import gettext
 
-# This will generate an xgettext warning
-my_string = gettext("This string contain two placeholders: %s and %s" % ('a', 'b'))
+# This will generate an xgettext "Empty msgid" warning.
+my_string = gettext('')