Bug 1605991 - Avoid coercing null manifest colors into "null" r=geckoview-reviewers,rbarker

Differential Revision: https://phabricator.services.mozilla.com/D58255

--HG--
extra : moz-landing-system : lando

mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoSession.java

@@ -6046,6 +6046,10 @@ public class GeckoSession implements Parcelable {
     }
 
     private static void fixupManifestColor(final JSONObject manifest, final String name) throws JSONException {
+        if (manifest.isNull(name)) {
+            return;
+        }
+
         manifest.put(name, rgbaToArgb(manifest.getString(name)));
     }