ci: Fix TypoError error when traces in traces.yml is an empty list
v2: Python's nitpick (Andres)
In case of an empty list of traces, the results.yml will contain an empty
curly braces. In YAML, an associative array can also be specified by text
enclosed in curly braces ({}),
This commit also adds the corresponding test to check the behavior of
tracie when no traces are added in the traces.yml file.
Signed-off-by: Pablo Saavedra <psaavedra@igalia.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4916>