Correcly handle nulls during path traversal

If we run into a null, just treat that the same as any other missing
field -- so if ignoreMissing is true, then we should not throw an
exception.

Author: joegallo

server/src/main/java/org/elasticsearch/ingest/IngestDocument.java

@@ -345,7 +345,7 @@ public void removeField(String path, boolean ignoreMissing) {
         }
 
         String leafKey = fieldPath.pathElements[fieldPath.pathElements.length - 1];
-        if (context == null) {
+        if (context == null && ignoreMissing == false) {
             throw new IllegalArgumentException(Errors.cannotRemove(path, leafKey, null));
         } else if (context instanceof IngestCtxMap map) { // optimization: handle IngestCtxMap separately from Map
             if (map.containsKey(leafKey)) {