Workboard: fix regression caused by tags= prefiller

Summary:
This change fixes a regression introduced here:

rP5e2b3677157889104a7e540d7772a04f13164037

Thank you to the user @dadalha for auditing that commit
and sharing a stack trace to easily fix the issue.

In short, if you see this exception you are affected:

```
EXCEPTION: (InvalidArgumentException)
Value provided to "replaceQueryParam()" for key "tags" is NULL.
Use "removeQueryParam()" to remove a query parameter.
at [<arcanist>/src/parser/PhutilURI.php:341]
```

This change just fixes that specific problem.

Closes T15221

Test Plan:
1. Create a new Project
2. Create a new Milestone inside
3. Create the Milestone's Workboard
4. Visit the Milestone's Workboard and note that now it works

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: speck, tobiaswiese, Matthew, Cigaryno, dadalha

Tags: #workboard

Maniphest Tasks: T15221

Differential Revision: https://we.phorge.it/D25103

src/applications/project/controller/PhabricatorProjectBoardViewController.php

@@ -693,11 +693,19 @@ final class PhabricatorProjectBoardViewController
       ->newCreateActionSpecifications(array());
 
     foreach ($specs as $spec) {
+
+      // Prefill tags= when you open the Column menu
+      // https://we.phorge.it/T15147
+      $spec_href = new PhutilURI($spec['uri']);
+      $spec_slug = $project->getPrimarySlug();
+      if ($spec_slug !== null) {
+        $spec_href->replaceQueryParam('tags', $spec_slug);
+      }
+
       $column_items[] = id(new PhabricatorActionView())
         ->setIcon($spec['icon'])
         ->setName($spec['name'])
-        ->setHref(id(new PhutilURI($spec['uri']))
-          ->replaceQueryParam('tags', $project->getPrimarySlug()))
+        ->setHref($spec_href)
         ->setDisabled($spec['disabled'])
         ->addSigil('column-add-task')
         ->setMetadata(