Fix IX bidder crash

[zxPhoenix]

🔧 Type of changes

• new bid adapter
• bid adapter update
• new feature
• new analytics adapter
• new module
• module update
• bugfix
• documentation
• configuration
• dependency update
• tech debt (test coverage, refactorings, etc.)

✨ What's the context?

Fix IX bidder's crash with Nullpointer at merging NativeImpTrackers for adm when Imptrackers are absent.

🧠 Rationale behind the change

It makes auction fail.
Stacktrace:

java.lang.NullPointerException: Cannot invoke "java.util.List.stream()" because the return value of "com.iab.openrtb.response.Response.getImptrackers()" is null
	at org.prebid.server.bidder.ix.IxBidder.mergeNativeImpTrackers(IxBidder.java:392)
	at org.prebid.server.bidder.ix.IxBidder.mergeNativeImpTrackers(IxBidder.java:382)
	at org.prebid.server.bidder.ix.IxBidder.updateBidAdmWithNativeAttributes(IxBidder.java:364)
	at org.prebid.server.bidder.ix.IxBidder.toBidderBid(IxBidder.java:281)
	at org.prebid.server.bidder.ix.IxBidder.lambda$extractBids$2(IxBidder.java:261)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197)
	at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1708)
	at java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:762)
	at java.base/java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:276)
	at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197)
	at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1708)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:575)
	at java.base/java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:260)
	at java.base/java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:616)
	at java.base/java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:622)
	at java.base/java.util.stream.ReferencePipeline.toList(ReferencePipeline.java:627)
	at org.prebid.server.bidder.ix.IxBidder.extractBids(IxBidder.java:263)
	at org.prebid.server.bidder.ix.IxBidder.makeBidderResponse(IxBidder.java:238)
	at org.prebid.server.bidder.HttpBidderRequester.makeBids(HttpBidderRequester.java:318)
	at org.prebid.server.bidder.HttpBidderRequester.processHttpCall(HttpBidderRequester.java:295)
	at org.prebid.server.bidder.HttpBidderRequester.lambda$requestBids$2(HttpBidderRequester.java:130)...```

### 🔎 New Bid Adapter Checklist
- [ ] verify email contact works
- [ ] NO fully dynamic hostnames
- [ ] geographic host parameters are NOT required
- [ ] direct use of HTTP is prohibited - *implement an existing Bidder interface that will do all the job*
- [ ] if the ORTB is just forwarded to the endpoint, use the generic adapter - *define the new adapter as the alias of the generic adapter*
- [ ] cover an adapter configuration with an integration test

### 🧪 Test plan
Covered by unit tests.

### 🏎 Quality check
- [x] Are your changes following [our code style guidelines](https://github.com/prebid/prebid-server-java/blob/master/docs/developers/code-style.md)?
- [ ] Are there any breaking changes in your code?
- [x] Does your test coverage exceed 90%?
- [ ] Are there any erroneous console logs, debuggers or leftover code in your changes?