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