I'm not sure that you can leave out SOP and EOP (i.e. not use them) as Tricky says. I couldn't find the document for the older version of the core you seem to be using, but the current version of the FFT user guide (page 3-19, Table 3-8) indicates that an error condition is detected if they are not used and you start streaming input data with a sink_valid. It also mentions if you don't follow the SOP/EOP protocol then an error occurs and the behavior is undefined.
According to the document set SOP high for one clock at the start of the data indicated by VALID and set EOP high for one clock on the last data of a block.