[sonic-yang] Backlinks are none when there is 0 backlinks

[ghooo]

Why I did it

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

How I did it

Allow backlinks to be None, so we get an empty list as a return from find_data_dependencies

How to verify it

    def test_find_leaf_dependencies_no_schema_dependencies(self):
        sy = sonic_yang.SonicYang(YANG_DIR)
        sy.loadYangModel()
        sy.loadData(json.loads("""
{
    "PORT": {
        "Ethernet0": {
            "alias":"eth0",
            "lanes": "65",
            "speed": "100000"
        }
    }
}
            """))

        # the PORT alias has no leafrefs pointing to it
        xpath = "/sonic-port:sonic-port/PORT/PORT_LIST[name='Ethernet0']/alias"
        deps = sy.find_data_dependencies(xpath)

        self.assertEqual(0, len(deps))

(added UT for the same idea)

Which release branch to backport (provide reason below if selected)

[qiluo-msft]

@praveen-li Could you help review

[praveen-li]

LGTM, fix the alignment if wrong, before merge. Thx.