J1939 can legitimately have 0 data bytes so don't give the impression data is "missing".

Change-Id: I285ba0d1aba3cbe230831ed41e7bcf8d6d9f74f3
Reviewed-on: https://code.wireshark.org/review/15109
Reviewed-by: Michael Mann <mmann78@netscape.net>

epan/dissectors/packet-j1939.c

@@ -333,7 +333,7 @@ void proto_register_j1939(void)
         },
         { &hf_j1939_data,
             {"Data", "j1939.data",
-            FT_BYTES, BASE_NONE, NULL, 0x0, NULL, HFILL }
+            FT_BYTES, BASE_NONE|BASE_ALLOW_ZERO, NULL, 0x0, NULL, HFILL }
         },
     };