Fix sporadic error about broken pipe when checking recent SRs

This code sometimes runs into:

```
Exception ignored in: <osc.util.safewriter.SafeWriter object at 0x7fbe42c17860>
BrokenPipeError: [Errno 32] Broken pipe
Retrying up to 3 more times after sleeping 3s …
```

This is easily reproducible by adding the condition into a separate script
and running it a few times. The additional `cat > /dev/null` which this
change introduces seems to fix the issue.

Related ticket: https://progress.opensuse.org/issues/167395

os-autoinst-obs-auto-submit

@@ -182,7 +182,13 @@ if [[ -e job_post_skip_submission ]]; then
 fi
 
 # throttle number of submissions to allow only one SR within a certain number of days
-if [[ $throttle_days != 0 ]] && $osc request list --project "$dst_project" --type submit --state new,review --mine --days "$throttle_days" | grep --quiet 'Created by'; then
+if [[ $throttle_days != 0 ]] && $osc request list --project "$dst_project" --type submit --state new,review --mine --days "$throttle_days" \
+    | {
+        grep --quiet 'Created by'
+        rc=$?
+        cat > /dev/null
+        exit $rc
+    }; then
     echo "Skipping submission, there is still a pending SR younger than $throttle_days days."
     exit 0
 fi