feat(frontend): illustrated null states and author rewards fallback

Add aksrt_get_null / widget null helpers with null.svg, use them in
sidebar widgets, comments empty, and related posts; when PM is off show
post rewards entry on the single author bar.

assets/css/frontend/widgets/part-2.css

@@ -298,3 +298,15 @@
 }
 
 /* ============================================================
+
+
+.aksrt-null--widget {
+  padding: 12px 0 !important;
+}
+.aksrt-null--widget .aksrt-null__img {
+  margin: 0 auto;
+  display: block;
+}
+.aksrt-null .separator {
+  border: 0;
+}