parser: check empty arg

src/main/java/io/socket/engineio/parser/Parser.java

@@ -76,6 +76,10 @@ public class Parser {
     }
 
     public static Packet<String> decodePacket(String data, boolean utf8decode) {
+        if (data == null) {
+            return err;
+        }
+
         int type;
         try {
             type = Character.getNumericValue(data.charAt(0));

src/test/java/io/socket/engineio/parser/ParserTest.java

@@ -156,6 +156,13 @@ public class ParserTest {
         });
     }
 
+    @Test
+    public void decodeEmptyPayload() {
+        Packet<String> p = decodePacket((String)null);
+        assertThat(p.type, is(Packet.ERROR));
+        assertThat(p.data, is(ERROR_DATA));
+    }
+
     @Test
     public void decodeBadFormat() {
         Packet<String> p = decodePacket(":::");