Remove null bytes in the SNMP test (sonic-net#17321)

Author: weiguo-nvidia

tests/snmp/test_snmp_phy_entity.py

@@ -827,7 +827,7 @@ def run_cmd(cmd):
         if not output:
             return {}
         # fix to make literal_eval() work with nested dictionaries
-        content = output.replace("'{", '"{').replace("}'", '}"')
+        content = output.replace('\x00', '').replace("'{", '"{').replace("}'", '}"')
         return ast.literal_eval(content)
 
     if duthost.is_multi_asic: