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