Fix using SetNewLenNode on empty storage

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/common/SequenceStorageNodes.java

@@ -3320,6 +3320,11 @@ static void doBasic(ArrayBasedSequenceStorage s, int len) {
             s.setNewLength(len);
         }
 
+        @Specialization
+        static void doEmpty(@SuppressWarnings("unused") EmptySequenceStorage s, int len) {
+            assert len == 0;
+        }
+
         @Specialization
         static void doNative(NativeSequenceStorage s, int len,
                         @Cached(inline = false) SetNativeLenNode setLen) {