tests/strfuncs: Do not compare string literal with pointers

Otherwise clang would complain:

  ../glib/tests/strfuncs.c:2603:32: warning: result of comparison
    against a string literal is unspecified (use an explicit string
    comparison function instead) [-Wstring-compare]
    g_assert_true ((gpointer)str != (gpointer)"");
                                 ^  ~~~~~~~~~~~~
  ../glib/gtestutils.h:187:59: note: expanded from macro 'g_assert_true'
                                               if G_LIKELY (expr) ; else \
                                                            ^~~~
  ../glib/gmacros.h:1186:59: note: expanded from macro 'G_LIKELY'
  #define G_LIKELY(expr) (__builtin_expect (_G_BOOLEAN_EXPR(expr), 1))
                                                            ^~~~
  ../glib/gmacros.h:1180:8: note: expanded from macro '_G_BOOLEAN_EXPR'
     if (expr)