Avalon Streaming Sink / Source Interface Signals

The empty signal is used to tell a sink port how many symbols sent from the source port are invalid.

So if you data width is 32 bits wide and your symbols are bytes (8 bits each) you would use an empty width of 2 bits. So it could be that your custom component with the source port isn't declaring the empty signal to be wide enough. Typically with video you treat RGB, YCbCr, etc... components as symbols so I suspect you'll probably need to do the same.

The VIP documentation should tell you what the streaming format is for that component and you would just make sure that your custom component feeding the data matches it.