Eliminate constant expressions from schema export

```ts
const executor = new PgExecutor({
  name: "main",
  context() {
    const ctx = context();
    return object({
      pgSettings: "pgSettings" != null ? ctx.get("pgSettings") : constant(null),
      withPgClient: ctx.get("withPgClient")
    });
  }
});
```

The expression `"pgSettings" != null ? ctx.get("pgSettings") : constant(null)` can be replaced with just `ctx.get("pgSettings")`.