Add fallback in case there is no thumbnail #180

Job	Run time
call-moodle-ci-workflow / static (8.2, MOODLE_403_STABLE, mariadb)	1m 13s
call-moodle-ci-workflow / test (MOODLE_401_STABLE/8.0/mariadb)	2m 31s
call-moodle-ci-workflow / test (MOODLE_401_STABLE/8.0/pgsql)	2m 42s
call-moodle-ci-workflow / test (MOODLE_402_STABLE/8.0/mariadb)	2m 34s
call-moodle-ci-workflow / test (MOODLE_402_STABLE/8.0/pgsql)	2m 7s
call-moodle-ci-workflow / test (MOODLE_403_STABLE/8.0/mariadb)	2m 36s
call-moodle-ci-workflow / test (MOODLE_403_STABLE/8.0/pgsql)	2m 28s
call-moodle-ci-workflow / test (MOODLE_401_STABLE/8.1/mariadb)	1m 52s
call-moodle-ci-workflow / test (MOODLE_401_STABLE/8.1/pgsql)	1m 46s
call-moodle-ci-workflow / test (MOODLE_402_STABLE/8.1/mariadb)	1m 49s
call-moodle-ci-workflow / test (MOODLE_402_STABLE/8.1/pgsql)	1m 50s
call-moodle-ci-workflow / test (MOODLE_403_STABLE/8.1/mariadb)	1m 50s
call-moodle-ci-workflow / test (MOODLE_403_STABLE/8.1/pgsql)	2m 2s
call-moodle-ci-workflow / test (MOODLE_402_STABLE/8.2/mariadb)	1m 49s
call-moodle-ci-workflow / test (MOODLE_402_STABLE/8.2/pgsql)	2m 14s
call-moodle-ci-workflow / test (MOODLE_403_STABLE/8.2/mariadb)	2m 41s
call-moodle-ci-workflow / test (MOODLE_403_STABLE/8.2/pgsql)	1m 52s
	35m 56s

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