Fixup 01.basic.t for v5.29.9 Variable length lookbehind support

Now implemented and the not-found return value changed from undef to ''

diff -up re-engine-RE2-0.13/t/00.re2-tests.t.530 re-engine-RE2-0.13/t/00.re2-tests.t
diff -up re-engine-RE2-0.13/t/01.basic.t.530 re-engine-RE2-0.13/t/01.basic.t
--- re-engine-RE2-0.13/t/01.basic.t.530	2019-05-16 10:58:24.910478530 +0200
+++ re-engine-RE2-0.13/t/01.basic.t	2019-05-16 11:00:22.200612388 +0200
@@ -9,7 +9,7 @@ ok("Hello, world" =~ /Hello, (world)/);
 is($1, 'world');
 
 no re::engine::RE2;
-is(eval '"Hello, world" =~ /(?<=Moose|Mo), (world)/', undef);
+ok(!eval '"Hello, world" =~ /(?<=Moose|Mo), (world)/'); # undef or ''
 
 if (fork) {
     ok(1);