Make variables optional on query hooks

When using `usePreloadedQuery` and `useLazyLoadQuery` it seems odd to require passing `variables` param since not every query requires variables.

Similar to the `options`, we could make `variables` optional to provide a better DX.

**Before:**
```js
usePreloadedQuery(query, {})
```

**After:**
```js
usePreloadedQuery(query)
```