Fix exception when api_extensions is set to empty

test.is_extension_enabled() is not checking if
config_dict[service] list is empty

Added a check if config_dict[service] is empty the
function will return False which means no extensions
are enabled

Change-Id: I2ee64f205c393637b5fc65897f1489292781c0be
Closes-Bug: 1342516
Merge "Fix exception when api_extensions is set to empty"