Fix GCC "warning: ā€˜%sā€™ directive argument is null" in tests.

* subversion/tests/svn_test.h
  (SVN_TEST_STRING_ASSERT): Don't pass NULL as strings into formatters.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1888590 13f79535-47bb-0310-9956-ffa450edef68
1 file changed