systems: Prevent empty UUID and SubModel

The afterGetUUID and afterGetInventory handlers set UUID and
SubModel properties without validating D-Bus returned values are
non-empty. Per Redfish schema, UUID must be a valid RFC4122
identifier and empty strings are invalid. This adds empty string
checks to prevent setting invalid property values in the Redfish
response.

When D-Bus returns empty strings for these properties (which can
occur during early boot or with certain hardware configurations),
they will now be omitted from the response rather than returned
as empty values.

Tested:
- Verified properties are omitted when D-Bus returns empty string
- Verified properties are correctly set when valid values present
- No functional change when valid non-empty values are returned

Change-Id: I80b4f8588cb065d25097f4fd3712987adef820c0
Signed-off-by: Vinothkumar Shanmugavel <vinothkumars@ami.com>