fix(kubernetes): make SCT support seedless scylla using operator

operator v1.4.x switched to usage of seedless approach for scylla
and now we should not fail test run if no seed nodes are found.

So, make it be smart enough and do not fail when seed nodes are
not defined, but still allow it to be set.
It will allow us to run old and new operator versions using the same
code.

sdcm/cluster_k8s/__init__.py

@@ -1970,9 +1970,15 @@ def node_setup(self, node: BaseScyllaPodContainer, verbose: bool = False, timeou
 
         if TestConfig.MULTI_REGION:
             node.datacenter_setup(self.datacenter)  # pylint: disable=no-member
+        try:
+            # NOTE: case of seedful scylla (operator v1.3.0-)
+            seed_nodes = ','.join(self.seed_nodes_ips)
+        except AssertionError:
+            # NOTE: case of seedless scylla (operator v1.4.0+)
+            seed_nodes = None
         self.node_config_setup(
             node,
-            seed_address=','.join(self.seed_nodes_ips),
+            seed_address=seed_nodes,
             endpoint_snitch=self.get_endpoint_snitch(),
         )