metadata: fix a crash if the description is invalid
metadata: fix a crash if the description is invalid

When generating the description.yaml from a v2 only dispatcher, the resulting
file can't be loaded properly on a classical server.

Change-Id: I67b8418e1fa48053e5b9a538343f65296b0d22f7

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server