[pretest] update collect_dut_lossless_prio method

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### Description of PR
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Summary:
Fix method collect_dut_lossless_prio.
The pfc_enable parameter can be empty.
Without this change we can get error:
```
>   result = [int(x) for x in port_qos_map[intf]['pfc_enable'].split(',')]
E   ValueError: invalid literal for int() with base 10: ''
```

PRs with related changes:
sonic-net/sonic-buildimage#22252
sonic-net/sonic-buildimage#22067

### Type of change

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

### Back port request
- [ ] 202012
- [ ] 202205
- [ ] 202305
- [ ] 202311
- [ ] 202405
- [x] 202411

### Approach
#### What is the motivation for this PR?
stable test_pretest.py

#### How did you do it?
Used method `get`

#### How did you verify/test it?
Test passed after changes

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Author: mssonicbld

tests/test_pretest.py

@@ -208,7 +208,7 @@ def collect_dut_lossless_prio(dut):
 
     """ Here we assume all the ports have the same lossless priorities """
     intf = list(port_qos_map.keys())[0]
-    if 'pfc_enable' not in port_qos_map[intf]:
+    if not port_qos_map[intf].get('pfc_enable'):
         return []
 
     result = [int(x) for x in port_qos_map[intf]['pfc_enable'].split(',')]