libweston: set default monitor strings

Allow passing NULL to monitor string in order to set the default
"unknown" strings. This allows a head to be initialized with the default
strings, meaning that it will no longer be mandatory for a backend to
call weston_head_set_monitor_strings(). In DRM-backend case this makes
future changes more convenient.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>