[generic_config_updater]Add empty input fixture to verify configDB be…

…fore GCU tests (#5506)

Summary: Add empty input fixture for GCU tests to check if current configDB is valid and GCU feature

What is the motivation for this PR?
During PR test, it will fail on all GCU tests if the configDB does not pass yang validation.
This empty input fixture is to verify if there is invalid config before test.

How did you do it?
Add empty input fixture.

How did you verify/test it?
Run test of sonic-mgmt/tests/generic_config_updater/test_*.py on KVM with invalid config
to see if it fail immediately.

tests/generic_config_updater/conftest.py

@@ -1,6 +1,8 @@
 import pytest
 
 from tests.common.utilities import skip_release
+from tests.generic_config_updater.gu_utils import apply_patch
+from tests.generic_config_updater.gu_utils import generate_tmpfile, delete_tmpfile
 
 @pytest.fixture(autouse=True)
 def ignore_expected_loganalyzer_exceptions(duthost, loganalyzer):
@@ -53,3 +55,28 @@ def cfg_facts(duthosts, rand_one_dut_hostname):
     """
     duthost = duthosts[rand_one_dut_hostname]
     return duthost.config_facts(host=duthost.hostname, source="persistent")['ansible_facts']
+
+
+@pytest.fixture(scope="module", autouse=True)
+def verify_configdb_with_empty_input(duthost):
+    """Fail immediately if empty input test failure
+
+    Args:
+        duthost: Hostname of DUT.
+
+    Returns:
+        None.
+    """
+    json_patch = []
+    tmpfile = generate_tmpfile(duthost)
+
+    try:
+        output = apply_patch(duthost, json_data=json_patch, dest_file=tmpfile)
+        if output['rc'] or "Patch applied successfully" not in output['stdout']:
+            pytest.fail(
+                "SETUP FAILURE: ConfigDB fail to validate Yang. rc:{} msg:{}"
+                .format(output['rc'], output['stdout'])
+            )
+
+    finally:
+        delete_tmpfile(duthost, tmpfile)