[graph] driveItems listing results in panic

[dschmidt]

Describe the bug

A clear and concise description of what the bug is.

Steps to reproduce

1. Make sure there is at least one file in your personal space
2. http --verify no --auth admin:admin GET 'https://localhost:9200/graph/v1.0/me/drive/root/children'
3. 💥

Expected behavior

listing works

Actual behavior

502 Bad Gateway cause the handler panics in

ocis/services/graph/pkg/service/v0/driveitems.go

Line 858 in 52bb1a1

parentRef.SetDriveType(res.Space.SpaceType)

because res.Space is nil

Setup

Please describe how you started the server and provide a list of relevant environment variables or configuration files.

OCIS_XXX=somevalue
OCIS_YYY=somevalue
PROXY_XXX=somevalue

Additional context

Add any other context about the problem here.