filters: fix objc null checks to account for collections (#1148)
filters: fix objc null checks to account for collections (#1148)

Description: Objective-C collections that don't allow nil use NSNull as a sentinel for missing values. Because these mappers may be used on values that come from collections, this fixes their null checks to be sensitive to NSNull as well as nil.
Risk Level: Low
Testing: Local

Signed-off-by: Mike Schore <270e364e708f2af319f244c09ceba52a03dbd980@gmail.com>
envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile