[pretest] update collect_dut_lossless_prio method (#17907)

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

Signed-off-by: AntonHryshchuk <[email protected]>

Author: AntonHryshchuk

tests/test_pretest.py

@@ -226,7 +226,7 @@ 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(',')]