Bug fix for null values in schema.

This currently throws an error:

```javascript
var data = null;
var schema = { "default": null };
tv4.validate(data, schema);
```

And traced it to normSchema and ValidatorContext.searchSchemas which break when `schema` is null or undefined, which can happen for example if a schema's `default` property has those as the value, or nested within. Added sanity checks and tests for both to solve the issue.