Return an empty dict if no data
Return an empty dict if no data

Avoids a HTTP500 on slow instances where the file
may be created before data is written, causing the
YAML parser to return None.

Change-Id: I13b92941f3e368839a9665fe3197c706babd9335

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