Fix check config packages key error (#15840)

* Fix packages deletion in check_config script

* The config key for packages is not present if core config validation
  failed. We need to do a safe dict deletion using dict.pop.

* Add check_config test for bad core config

homeassistant/scripts/check_config.py

@@ -325,7 +325,7 @@ def check_ha_config_file(hass):
     # Merge packages
     merge_packages_config(
         hass, config, core_config.get(CONF_PACKAGES, {}), _pack_error)
-    del core_config[CONF_PACKAGES]
+    core_config.pop(CONF_PACKAGES, None)
 
     # Ensure we have no None values after merge
     for key, value in config.items():

tests/scripts/test_check_config.py

@@ -22,6 +22,12 @@ BASE_CONFIG = (
     '\n\n'
 )
 
+BAD_CORE_CONFIG = (
+    'homeassistant:\n'
+    '  unit_system: bad\n'
+    '\n\n'
+)
+
 
 def normalize_yaml_files(check_dict):
     """Remove configuration path from ['yaml_files']."""
@@ -47,6 +53,17 @@ class TestCheckConfig(unittest.TestCase):
         self.maxDiff = None  # pylint: disable=invalid-name
 
     # pylint: disable=no-self-use,invalid-name
+    @patch('os.path.isfile', return_value=True)
+    def test_bad_core_config(self, isfile_patch):
+        """Test a bad core config setup."""
+        files = {
+            YAML_CONFIG_FILE: BAD_CORE_CONFIG,
+        }
+        with patch_yaml_files(files):
+            res = check_config.check(get_test_config_dir())
+            assert res['except'].keys() == {'homeassistant'}
+            assert res['except']['homeassistant'][1] == {'unit_system': 'bad'}
+
     @patch('os.path.isfile', return_value=True)
     def test_config_platform_valid(self, isfile_patch):
         """Test a valid platform setup."""