fix(sqllab): Replace stringified 'null' schema column values with NULL (#18992)

Co-authored-by: John Bodley <john.bodley@airbnb.com>

superset/migrations/versions/b5a422d8e252_fix_query_and_saved_query_null_schema.py

@@ -0,0 +1,66 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+"""fix query and saved_query null schema
+
+Revision ID: b5a422d8e252
+Revises: b8d3a24d9131
+Create Date: 2022-03-02 09:20:02.919490
+
+"""
+
+from alembic import op
+from sqlalchemy import Column, Integer, String
+from sqlalchemy.ext.declarative import declarative_base
+
+from superset import db
+
+# revision identifiers, used by Alembic.
+revision = "b5a422d8e252"
+down_revision = "b8d3a24d9131"
+
+Base = declarative_base()
+
+
+class Query(Base):
+    __tablename__ = "query"
+
+    id = Column(Integer, primary_key=True)
+    schema = Column(String(256))
+
+
+class SavedQuery(Base):
+    __tablename__ = "saved_query"
+
+    id = Column(Integer, primary_key=True)
+    schema = Column(String(128))
+
+
+def upgrade():
+    bind = op.get_bind()
+    session = db.Session(bind=bind)
+
+    for model in (Query, SavedQuery):
+        for record in session.query(model).filter(model.schema == "null"):
+            record.schema = None
+
+        session.commit()
+
+    session.close()
+
+
+def downgrade():
+    pass