getActiveWorkspace throws TypeError when there is no current request

Problem/Motivation

If something asks for the active workspace before a request is pushed onto the RequestStack, getActiveWorkspace() passes NULL to WorkspaceNegotiatorInterface::applies(Request $request) and PHP throws a TypeError.
We hit this on a site with Workspaces enabled and Shield installed. After a cache clear, Shield's middleware looks up an alias early. The alias whitelist is empty, so WorkspacesAliasRepository::getBaseQuery() calls hasActiveWorkspace(), which crashes:
TypeError: QueryParameterWorkspaceNegotiator::applies(): Argument #1 ($request) must be of type Request, null given, called in WorkspaceManager.php

Steps to reproduce

1. Enable Workspaces and Shield.
2. Clear caches.
3. Load any page.

Proposed resolution

Return FALSE from getActiveWorkspace() when getCurrentRequest() is NULL.