Use new_interval to avoid offloading void XST data in first sync interval.

...ons/lofar2/libraries/sdp/src/vhdl/node_sdp_correlator.vhd

...2/libraries/sdp/src/vhdl/sdp_crosslets_subband_select.vhd

.../lofar2/libraries/sdp/src/vhdl/sdp_statistics_offload.vhd

...ofar2/libraries/sdp/tb/vhdl/tb_sdp_statistics_offload.vhd