Fix passing null to trim() error in DiffusionBrowseTableView render()

Summary:
When viewing the top level of a diffusion repository, $request->getPath() returns null, which then causes trim() to throw an error.

Fix is to update DiffusionRequest getPath() such that it returns '' instead of null, thus fixing it's return type.

Fixes T15522

Test Plan: Go to https://my.phorge.site/source/myrepo

Reviewers: O1 Blessed Committers, valerio.bozzolan, avivey

Reviewed By: O1 Blessed Committers, valerio.bozzolan, avivey

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15522

Differential Revision: https://we.phorge.it/D25329