Fix dropping ssh connections to non-provisioned terminating instances

[r4victor]

Follows #3936

Fixes an error when dropping ssh connection to terminated but not-yet-provisioned instances.

                    "/Users/r4victor/Projects/dstack/dstack/src/dstack/_internal/server/se
                    rvices/runner/pool.py", line 50, in from_jpd
                        assert jpd.hostname is not None and jpd.ssh_port is not None
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                    AssertionError