Validate `firstEvaluatedKey` and `lastEvaluatedKey`'s length

Here is the code that validates `firstEvaluatedKey` and `lastEvaluatedKey`.

```
export const txPaginationSchema = {
firstEvaluatedKey: Joi.array().items(Joi.number(), Joi.number()),
lastEvaluatedKey: Joi.array().items(Joi.number(), Joi.number())
};
```

The above condition does not check an input array's length. All of these inputs `[]`, `[1]`, `[1,2]` and `[1,2,3]` are valid in the current implementation. Only `[1,2]` should pass the validation and other `[]`, `[1]`, `[1,2,3]` should fail the validation.