Fix checking for restart files (NOAA-EMC#1186)

Undoes the portion of PR NOAA-EMC#1179 that caused a new bug while attempting to fix NOAA-EMC#1140, without removing the linter fixes. Instead `/dev/null` is 'searched' if `${RSTDIR_ATM}` is not defined. That situation will always result in zero files found, ensuring a rerun is not triggered.

Fixes NOAA-EMC#1140 
Fixes NOAA-EMC#1185
Moots NOAA-EMC#1190

ush/forecast_det.sh

@@ -40,7 +40,7 @@ FV3_GFS_det(){
   #-------------------------------------------------------
   # determine if restart IC exists to continue from a previous forecast
   RERUN="NO"
-  [[ ${CDUMP} = "gfs" ]] && filecount=$(find "${RSTDIR_ATM}" -type f | wc -l)
+  filecount=$(find "${RSTDIR_ATM:-/dev/null}" -type f | wc -l)
   if [ ${CDUMP} = "gfs" -a ${rst_invt1} -gt 0 -a ${FHMAX} -gt ${rst_invt1} -a ${filecount} -gt 10 ]; then
     reverse=$(echo "${restart_interval[@]} " | tac -s ' ')
     for xfh in ${reverse} ; do