🐛 Fixed 404 errors if navigation URLs are somehow blank

no issue

- in very rare circumstances it's possible that a navigation url in settings can be blank, we should not throw errors in this case as it appears as a theme/routing problem which is difficult to diagnose and much worse than simply not outputting a link class

core/frontend/helpers/link_class.js

@@ -18,7 +18,7 @@ module.exports = function link_class(options) { // eslint-disable-line camelcase
     // If the for attribute is present but empty, this is probably a dynamic data problem, hard for theme devs to track down
     // E.g. {{link_class for=slug}} in a context where slug returns an empty string
     // Error's here aren't useful (same as with empty get helper filters) so we fallback gracefully
-    if (!options.hash.for) {
+    if (!options.hash.for || options.hash.for.string === '') {
         options.hash.for = '';
     }
 

test/unit/helpers/link_class_spec.js

@@ -48,6 +48,12 @@ describe('{{link_class}} helper', function () {
             .should.eql('');
     });
 
+    it('silently accepts an empty SafeString', function () {
+        compile('{{link_class for=blank_safe_string}}')
+            .with({blank_safe_string: new handlebars.SafeString('')})
+            .should.eql('');
+    });
+
     describe('activeClass', function () {
         it('gets applied correctly', function () {
             // Test matching relative URL