NOS-225: Check for no packets received in sflow PTF test (sonic-net#1111)

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Summary:

Currently the test will fail with a KeyError somewhere random, because
it assumes it will always get at least one packet. But if the test is
badly failing, that might not be true. Better to have a clear error and
failure point.

Fixes # (issue)

### Type of change

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- [x] Bug fix
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- [x] Bug fix
- [ ] Testbed and Framework(new/improvement)
- [ ] New Test case
    - [ ] Skipped for non-supported platforms
- [ ] Test case improvement


### Back port request
- [x] 202505
- [x] 202511

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Author: anders-nexthop

ansible/roles/test/files/ptftests/py3/sflow_test.py

@@ -231,6 +231,7 @@ def analyze_counter_sample(self, data, collector, polling_int, port_sample):
     # ---------------------------------------------------------------------------
 
     def analyze_flow_sample(self, data, collector):
+        self.assertTrue(data.get('flow_port_count', False), "No packets collected on any interface")
         logging.info("packets collected from interfaces ifindex : %s" %
                      data['flow_port_count'])
         logging.info("Expected number of packets from each port : %s to %s" % (