Improve support for conditionally generated files

There's a small set of summary files that may be generated, but sometimes there is no significant content for these files. In these cases, sometimes empty files are generated; sometimes the files are not generated at all.

This should be made more consistent.

One complication is that there may be references to these files, such as in the Navigation bar and from the Help page. These references need to be taken into account if the files are not generated.

A followup is that if/when we have better more consistent handling, it will be possible to improve the Help page, providing details for optional files when they are present, such as the System Properties file, which is currently not listed on the Help page.