core: tpgt should not be sent when empty

Ensure 'tpgt' isn't sent as an empty string as VDSM expects an integer.
I.e. 'tpgt' is optional, but if specified, it must be an integer.

Vdsm API:
"
{'type': 'IscsiConnectionParameters',
 'data': {'portal': 'IscsiPortal', 'iqn': 'str', '*tpgt': 'int',
          '*iface': 'str', '*credentials': 'IscsiCredentials'}}

@tpgt: #optional The Target Portal Group Tag to use
"

Change-Id: I79b6b80fafff7b87700e4e340eacff6cd0c43f30
Bug-Url: https://bugzilla.redhat.com/1160785
Signed-off-by: Daniel Erez <derez@redhat.com>