Allow totally empty lines of diff content

Empty diff lines had been coerced to non-breaking spaces in order to
preserve blank lines when copying to the clipboard [1]. Since then, copy
was upgraded to traverse the diff object rather than the DOM [2],
thereby obviating the need for non-breaking space to preserve blank
lines.

Remove the non-breaking space.

[1] 613b49c6
[2] 9fcec743

Bug: Issue 4854
Change-Id: I4039c76d996ed97e9fb237f2f6ca20f70be77675

polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.js

@@ -384,9 +384,6 @@
     var text = line.text;
     if (line.type !== GrDiffLine.Type.BLANK) {
       td.classList.add('content');
-      if (!text) {
-        text = '\xa0';
-      }
     }
     td.classList.add(line.type);
     var html = util.escapeHTML(text);