Pubmatic adapter seems to crash occasionally on invalid native ads

[justadreamer]

Background

Testing native ads in a publisher Android app working against a PBS-Java we observed occasional /auction 500 response with this error:

Critical error while running the auction: Cannot invoke "com.fasterxml.jackson.databind.JsonNode.isMissingNode()" because "nativeNode" is null

a quick search suggests this part of Pubmatic validation code:

prebid-server-java/src/main/java/org/prebid/server/bidder/pubmatic/PubmaticBidder.java

Lines 597 to 600 in d02bd51

	final JsonNode nativeNode = admNode.get("native");
	if (!nativeNode.isMissingNode()) {
	return nativeNode.toString();
	}

Suggested solution

Perhaps a prior null check of nativeNode would be a good defense - as in some cases it may be missing..

[AntoxaAntoxic]

Done by #3918