Native ad crash in 2.1.7

[ValentinPostindustria]

Describe the bug
Native ad without impression trackers crashes the app. It seems this happened after the PR merge #705.

Additional context
It happens when adm doesn't have eventrackers field and imp_size array is null.

Logcat:

AndroidRuntime           E  FATAL EXCEPTION: main
                            Process: com.magnite.demand.manager.kotlin, PID: 8657
                            java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
                            	at org.prebid.mobile.PrebidNativeAd.createImpressionTrackers(PrebidNativeAd.java:369)
                            	at org.prebid.mobile.PrebidNativeAd.registerView(PrebidNativeAd.java:350)
                            	at com.magnite.demand.manager.kotlin.activities.examples.gam.MultiformatBannerNativeInAppActivity.inflatePrebidNativeAd(MultiformatBannerNativeInAppActivity.kt:212)
                            	at com.magnite.demand.manager.kotlin.activities.examples.gam.MultiformatBannerNativeInAppActivity.access$inflatePrebidNativeAd(MultiformatBannerNativeInAppActivity.kt:30)
                            	at com.magnite.demand.manager.kotlin.activities.examples.gam.MultiformatBannerNativeInAppActivity$showPrebidNativeAd$1.onPrebidNativeLoaded(MultiformatBannerNativeInAppActivity.kt:181)
                            	at org.prebid.mobile.addendum.AdViewUtils.findNativeInGAMCustomTemplateAd(AdViewUtils.java:433)
                            	at org.prebid.mobile.addendum.AdViewUtils.findNative(AdViewUtils.java:411)