Fix a crash when using --no-generate-docs

[jdkoren]

Fixes a crash when accessing documentationAsHtml directly. Most usages first check hasDocumentation, but nothing enforces this. Notably, EmptyGenerator accesses this property without checking first (you can reproduce the crash by passing --no-generate-docs).

[jcollins-g]

I think a better solution would be to move this to _parseDocumentation and return ([], false) as the value of the tuple. The renderers should be able to handle that as input, and there's less special casing that way.

[jdkoren]

I think a better solution would be to move this to _parseDocumentation and return ([], false) as the value of the tuple. The renderers should be able to handle that as input, and there's less special casing that way.

This works, too. In that case I added a check in the renderer so it skips creating further objects.