Decorate the number line element of a blank side

The blank side of a side-by-side diff still needs the
normal class decoration, including the 'left' or 'right'
CSS class, otherwise it won't be affected by the "hide-left"
action to hide one side of the diff.

Bug: Issue 14104
Change-Id: I3c02aad282d2fb5dbd9259f815ac41cc975da644
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.js b/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.js
index 948002a..42fd6ea 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.js
@@ -294,6 +294,7 @@
     line, number, type, side) {
   const td = this._createElement('td');
   if (line.type === GrDiffLine.Type.BLANK) {
+    td.classList.add(side);
     return td;
   }
   if (line.type === GrDiffLine.Type.CONTEXT_CONTROL) {