rep_hist_desc_stats_write() segfaults on empty stats
The fix for legacy/trac#5508 (moved) wasn't complete -- returning NULL from rep_hist_format_desc_stats() makes rep_hist_desc_stats_write() segfault.
rep_hist_format_desc_stats() already returned NULL when !start_of_served_descs_stats_interval but that probably doesn't happen very often.
NOTE: This code is run only by bridge authoritatives.