check-sr-results.yaml: workaround missing map.log

The `map.log' file is not always produced during ACS run (see #41).

This is annoying, but not part of the compliance. Thus, until this is
properly understood workaround the issue by making this file optional.

[Backport commit 6306b2ee.]
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>