Make an empty hash shift return nil.

Author: itarato

src/main/java/org/truffleruby/core/hash/HashNodes.java

@@ -511,9 +511,9 @@ protected Object setDefault(RubyHash hash, Object defaultValue,
     public abstract static class ShiftNode extends CoreMethodArrayArgumentsNode {
 
         @Specialization(guards = "hash.empty()")
-        protected Object shiftEmpty(RubyHash hash,
+        protected Nil shiftEmpty(RubyHash hash,
                 @Cached DispatchNode callDefault) {
-            return callDefault.call(hash, "default", nil);
+            return nil;
         }
 
         @Specialization(guards = "!hash.empty()", limit = "hashStrategyLimit()")