mon/PGMap.cc: don't output header for pg dump_stuck if nothing stuck

Formatted output is already correct (no header)

Fixes: #4577
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #680 from dmick/next

mon/PGMap.cc: don't output header for pg dump_stuck if nothing stuck

Reviewed-by: Sage Weil <sage@inktank.com>