Listing relation tuples where none match cause SQL error

[aeneasr]

Describe the bug

Instead of receiving an empty list, I get: The requested resource could not be found.

Reproducing the bug

	testhelpers.NewKetoServer(t, reg.Config())

	identityID := x.NewUUID()

	projects, err := reg.AccessService().ListAccessibleProjects(ctx, identityID) // could be any non-existing relationtuple
	require.NoError(t, err) // fails with: The requested resource could not be found
	assert.Empty(t, projects)

After the first relation has been added, this no longer happens.