Fixed #31615 -- Made migrations skip extension operations if not needed.

- Don't try to create an existing extension.
- Don't try to drop a nonexistent extension.

docs/ref/contrib/postgres/operations.txt

@@ -30,12 +30,19 @@ For example::
             ...
         ]
 
+Django checks that the extension already exists in the database and skips the
+migration if so.
+
 For most extensions, this requires a database user with superuser privileges.
 If the Django database user doesn't have the appropriate privileges, you'll
 have to create the extension outside of Django migrations with a user that has
 them. In that case, connect to your Django database and run the query
 ``CREATE EXTENSION IF NOT EXISTS hstore;``.
 
+.. versionchanged:: 3.2
+
+    In older versions, the existence of an extension isn't checked.
+
 .. currentmodule:: django.contrib.postgres.operations
 
 ``CreateExtension``