fix(sdcm/tester.py): Check if version_str is non-empty
fix(sdcm/tester.py): Check if version_str is non-empty

This corrects an issue where argus_get_scylla_version would fail if
BaseNode.get_scylla_binary_version() returns None. In this case it logs
a warning instead.

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