[sonic-yang] Backlinks are none when there is 0 backlinks (#8187)

Backlinks are none when there is 0 backlinks. without this change backlinks.number() 
throws an exception when backlinks==None.

Author: ghooo

src/sonic-yang-mgmt/sonic_yang.py

@@ -526,7 +526,7 @@ def find_data_dependencies(self, data_xpath):
 
             schema_node = ly.Schema_Node_Leaf(data_node.schema())
             backlinks = schema_node.backlinks()
-            if backlinks.number() > 0:
+            if backlinks is not None and backlinks.number() > 0:
                 for link in backlinks.schema():
                      node_set = node.find_path(link.path())
                      for data_set in node_set.data():

src/sonic-yang-mgmt/tests/libyang-python-tests/test_SonicYang.json

@@ -59,10 +59,12 @@
 
   "dependencies":[
 	  {"xpath":"/test-port:port/PORT/PORT_LIST[port_name='Ethernet8']/port_name",
-           "dependencies":
-                 ["/test-acl:acl/ACL_TABLE/ACL_TABLE_LIST[ACL_TABLE_NAME='PACL-V6']/ports[.='Ethernet8']",
-                  "/test-interface:interface/INTERFACE/INTERFACE_LIST[interface='Ethernet8'][ip-prefix='10.1.1.64/26']/interface",
-                  "/test-interface:interface/INTERFACE/INTERFACE_LIST[interface='Ethernet8'][ip-prefix='2000:f500:40:a749::/126']/interface"]}
+    "dependencies":
+          ["/test-acl:acl/ACL_TABLE/ACL_TABLE_LIST[ACL_TABLE_NAME='PACL-V6']/ports[.='Ethernet8']",
+           "/test-interface:interface/INTERFACE/INTERFACE_LIST[interface='Ethernet8'][ip-prefix='10.1.1.64/26']/interface",
+           "/test-interface:interface/INTERFACE/INTERFACE_LIST[interface='Ethernet8'][ip-prefix='2000:f500:40:a749::/126']/interface"]},
+    {"xpath":"/test-port:port/PORT/PORT_LIST[port_name='Ethernet8']/alias",
+     "dependencies":[]}
   ],
   "schema_dependencies":[
           {"xpath":"/test-port:port/test-port:PORT/test-port:PORT_LIST/test-port:port_name",