Bug 1801138 - Ensure empty label values don't collapse. r=dao

Add a zwsp character to ensure they're not considered an empty line.

Differential Revision: https://phabricator.services.mozilla.com/D162301

toolkit/content/xul.css

@@ -167,6 +167,10 @@ description[value]::before {
   display: block;
 }
 
+label[value=""]::before {
+  content: "\200B"; /* zwsp */
+}
+
 label[value][crop]::before,
 description[value][crop]::before {
   min-width: 0;