Use placeholder in change list size col

Specifically. uses the placeholder for unknown sizes in the change list.

Bug: Issue 8467
Change-Id: I6d1ae5c3f5eaa18906832a380313ed577ff87262

polygerrit-ui/app/elements/change-list/gr-change-list-item/gr-change-list-item.js

diff --git a/polygerrit-ui/app/elements/change-list/gr-change-list-item/gr-change-list-item.js b/polygerrit-ui/app/elements/change-list/gr-change-list-item/gr-change-list-item.js
index 5d7121a..9675f9a 100644
--- a/polygerrit-ui/app/elements/change-list/gr-change-list-item/gr-change-list-item.js
+++ b/polygerrit-ui/app/elements/change-list/gr-change-list-item/gr-change-list-item.js
@@ -45,6 +45,10 @@
         value: false,
       },
       showNumber: Boolean,
+      _changeSize: {
+        type: String,
+        computed: '_computeChangeSize(change)',
+      },
     },
 
     behaviors: [
@@ -153,7 +157,7 @@
     _computeChangeSize(change) {
       const delta = change.insertions + change.deletions;
       if (isNaN(delta) || delta === 0) {
-        return '🤷'; // Unknown
+        return null; // Unknown
       }
       if (delta < CHANGE_SIZE.XS) {
         return 'XS';