uninstall-backup-restore.sh failing on older oc client versions

With oc client version 4.16 installed the script fails with 
```
oc version: null
```

Solution replace this line 
```
[ -z "$OCVER_FULL"  ] && OCVER_FULL=$(oc version --client -o json | jq -r '.clientVersion.gitVersion')
```
with 
```
[ -z "$OCVER_FULL" -o "$OCVER_FULL" = "null" ] && OCVER_FULL=$(oc version --client -o json | jq -r '.clientVersion.gitVersion')
``` 
as null is not non-zero value 