Add ALTER for null storage tables.

dbms/src/Storages/StorageNull.cpp

 #include <Storages/StorageNull.h>
 #include <Storages/StorageFactory.h>
 
+#include <Interpreters/InterpreterAlterQuery.h>
+#include <Databases/IDatabase.h>
+
 #include <IO/WriteHelpers.h>
 
 
@@ -27,4 +30,15 @@ void registerStorageNull(StorageFactory & factory)
     });
 }
 
+void StorageNull::alter(const AlterCommands & params, const String & database_name, const String & table_name, const Context & context)
+{
+    auto lock = lockStructureForAlter(__PRETTY_FUNCTION__);
+
+    params.apply(columns, materialized_columns, alias_columns, column_defaults);
+
+    context.getDatabase(database_name)->alterTable(
+        context, table_name,
+        columns, materialized_columns, alias_columns, column_defaults, {});
+}
+
 }

dbms/src/Storages/StorageNull.h

@@ -43,6 +43,8 @@ public:
         name = new_table_name;
     }
 
+    void alter(const AlterCommands & params, const String & database_name, const String & table_name, const Context & context) override;
+
 private:
     String name;
     NamesAndTypesList columns;

dbms/tests/queries/0_stateless/00557_alter_null_storage_tables.reference

+x	UInt8		
+x	Int64	DEFAULT	toInt64(y)
+y	String		

dbms/tests/queries/0_stateless/00557_alter_null_storage_tables.sql

+DROP TABLE IF EXISTS test.null;
+
+CREATE TABLE test.null (x UInt8) ENGINE = Null;
+DESCRIBE TABLE test.null;
+
+ALTER TABLE test.null ADD COLUMN y String, MODIFY COLUMN x Int64 DEFAULT toInt64(y);
+DESCRIBE TABLE test.null;
+
+DROP TABLE test.null;