[CDF-353] - Dashboards._isParameterInModel should look for the parameter...

... in the correct context

  - Dashboards._isParameterInModel now looks for the parameter in the correct context
  - Parameters with null value are allowed and parameters with undefined value are overridable by addParameter
  - Dashboards.setParameter doesn't allow to set a parameter with an empty string as it's name
  - Dashboards.parameterModel and Dashboards.persistBookmarkables are only called by Dashboards.setParameter if the parameter is set in the target context object
  - Reviewed and added tests for Dashboards.addParameter and Dashboards.setParameter