openxOrtbBidAdapter: avoid error logging when response body is null (#8725)
openxOrtbBidAdapter: avoid error logging when response body is null (#8725)

```
prebid.js:5 Prebid openx ERROR: Bidder openx failed to interpret the server's response. Continuing without bids null TypeError: Cannot use 'in' operator to search for 'nbr' in at prebid.js:12:4571
```
Thanks @piotrj-rtbh
gumgum/Prebid.js,PubWise/Prebid.js,PubWise/Prebid.js,ImproveDigital/Prebid.js,HuddledMasses/Prebid.js,prebid/Prebid.js,prebid/Prebid.js,ImproveDigital/Prebid.js,gumgum/Prebid.js,HuddledMasses/Prebid.js,ImproveDigital/Prebid.js,PubWise/Prebid.js,gumgum/Prebid.js,prebid/Prebid.js,HuddledMasses/Prebid.js