Fix handling of bad pointers in arrays

When refactoring this code, I lost a behavior where a pointer to an
object that isn't found is dropped from the results (I had been
leaving an `undefined` in its place).

There's one tricky wrinkle, which is that if the value in the array is
`null`, it should be retained (see #2189 for
more details on why this is the expected behavior).

Author: noahsilas

src/RestQuery.js

@@ -963,9 +963,9 @@ function replacePointers(object, path, replace) {
     // this could be either a pointer or an array of pointers
     const pointerOrPointers = node[attrName];
     if (pointerOrPointers instanceof Array) {
-      node[attrName] = pointerOrPointers.map(
-        pointer => pointer && replace[pointer.objectId]
-      );
+      node[attrName] = pointerOrPointers
+        .map(pointer => pointer && replace[pointer.objectId])
+        .filter(pointer => pointer || pointer === null);
     } else if (pointerOrPointers && pointerOrPointers.__type === 'Pointer') {
       node[attrName] = replace[pointerOrPointers.objectId];
     }