Handle missing parameter type

[Cleecanth]

With the new nunjucks templates, the @parameter type no longer handles undefined values. Before this change, an undefined type just showed up as a blank string.

At the moment, parameter.type is passed to the display_as_type filter — where it tries to split it even if it's undefined.

I'm not sure if type is required or not on parameters, but in either case the absence of a type causes a pretty hard-to-track-down nunjucks render error.

I'm more than willing to put in a PR to fix this, but I don't want to assume behavior. If type is required for parameters, maybe this should be handled on the parser side of things. If it's not, then protecting the parameter.type logic is probably sufficient (in the same way that the @type template is protected).

Thanks for all your work. Let me know if you'd like me to add a PR.