In cache key use ObjectId.zeroId() if refs/meta/config is missing

Use ObjectId.zeroId() instead of null as meta config revision in the
cache key if the project has no refs/meta/config branch. This
prevents potential NullPointerExceptions later.

Change-Id: I810f1d626a8d57e20f4dbe493d4dd9a20e42f6ab
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>