fix(add_severity_limit_rules): to tollerate empty string
fix(add_severity_limit_rules): to tollerate empty string

max_events_severities set to "" in test_detault, which results in ['']
 after str_or_list conversion.
To make defaults work we need to make
 add_severity_limit_rules to skip empty strings

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests