WaResultsCollector: fix no kernel path case

If no kernel path has been defined, kernel_refs is empty and
df['kernel_sha1'].replace(kernel_refs) fails

Add sha1 as kernel ref in case no kernel path is provied

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
(cherry picked from commit c3741945)
[Minor merge conflict]
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>