[Config] Fix ssh proxy command to be null

sky/utils/schemas.py

@@ -323,7 +323,14 @@ def get_config_schema():
                             'type': 'object',
                             'required': [],
                             'additionalProperties': {
-                                'type': 'string',
+                                'anyOf': [
+                                    {
+                                        'type': 'string'
+                                    },
+                                    {
+                                        'type': 'null'
+                                    },
+                                ]
                             }
                         }]
                     },

tests/test_config.py

@@ -58,6 +58,29 @@ def test_empty_config(monkeypatch, tmp_path) -> None:
     _check_empty_config()
 
 
+def test_valid_null_proxy_config(monkeypatch, tmp_path) -> None:
+    """Test that the config is not loaded if the config file is empty."""
+    with open(tmp_path / 'valid.yaml', 'w') as f:
+        f.write(f"""\
+        aws:
+            instance_tags:
+                mytag: myvalue
+            ssh_proxy_command:
+                eu-west-1: null
+                us-east-1: null
+            use_internal_ips: true
+            vpc_name: abc
+
+        spot:
+            controller:
+                resources:
+                    disk_size: 256
+        """)
+    monkeypatch.setattr(skypilot_config, 'CONFIG_PATH', tmp_path / 'valid.yaml')
+    _reload_config()
+    _check_empty_config()
+
+
 def test_invalid_field_config(monkeypatch, tmp_path) -> None:
     """Test that the config is not loaded if the config file contains unknown field."""
     config_path = tmp_path / 'invalid.yaml'