Group update shouldn't require group name

Calling groups_update_group() (in group/lib.php) by passing a $data object that doesn't contain a 'name' property results in an 'undefined property' notice.

It should be possible to call the function without $data->name (e.g. if just the group description is being updated).

TO REPRODUCE THE ERROR

1. Comment the line 154 of the groups/tests/lib_test.php

   $data->name = 'Backend team';

2. Run unit tests on groups/tests/lib_test.php.
3. The unit test will fail and give this message:

   There was 1 error:

   1) core_group_lib_testcase::test_group_updated_event

   Undefined property: stdClass::$name