Fix last-launch time query failure behavior

If for whatever reason there is a failure in the query or search
for last-launch times, `$latest_launched` could be set to the current
time.  This will ultimately result in no instances being launched.  Fix
this by improved detection of an empty/null launch time in
`${launchtimes[@]}`.

Signed-off-by: Chris Evich <[email protected]>

mac_pw_pool/LaunchInstances.sh

@@ -112,9 +112,12 @@ if ! $AWS ec2 describe-instances --filters "${pw_filts[@]}" --query "$pw_query"
 $(<$inst_lt_f)"
 else
     declare -a launchtimes
-    if ! readarray -t launchtimes<<<$(json_query '.[]?' "$inst_lt_f"); then
+    if ! readarray -t launchtimes<<<$(json_query '.[]?' "$inst_lt_f") ||
+       [[ "${#launchtimes[@]}" -eq 0 ]] ||
+       [[ "${launchtimes[0]}" == "" ]]; then
         warn "Found no running instances, this should not happen."
     else
+        dbg "launchtimes=[${launchtimes[*]}]"
         for launch_time in "${launchtimes[@]}"; do
             # Assume launch_time is never malformed
             launched_hour=$(date -u -d "$launch_time" "$dcmpfmt")