"TypeError: expected string or buffer" when Validation Options are set in config file but not provided in the resource field

# Overview

As explained in the [Documentation](https://github.com/frictionlessdata/ckanext-validation#formats-to-validate) I set up a default value for the validation options in the configuration file.

I try to add a resource not providing any input in the resource field for validation options, and this leads to a Server error.


The Server error is "TypeError: expected string or buffer" generated from the following [line of code](https://github.com/frictionlessdata/ckanext-validation/blob/master/ckanext/validation/validators.py#L62).

So this should take into account the case where the value is null.

I've added this as a solution:

`

if value:

provided_options = json.loads(value)

default_options.update(provided_options)
`

This should solve the issue. Should I submit a PR?

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