Allow the vrm_id to be empty (issue #10)

The validation of the vrm_id would also triggers when not using the DESS
installation type. Allowing it to be empty instead of the 16 character
check passes the check when the type is not DESS.

src/nodes/vrm-api.html

@@ -33,7 +33,7 @@
             stats_end: {value: ""},
             use_utc: { value: false },
             // dess
-            vrm_id: {value: "", validate: RED.validators.regex(/^[0-9a-zA-Z]{1,16}$/), required: false},
+            vrm_id: {value: "", validate: RED.validators.regex(/^([0-9a-zA-Z]{1,16}|)$/),required: false},
             country: {value: "", required: false},
             b_max: {value: "", required: false},
             tb_max: {value: "", required: false},