Fix error when merging a single archive

[ffried]

Bug/issue #, if applicable:

Summary

Currently, when trying to merge a single doccarchive that contains either only references or only tutorials, the command fails because it tries to read from a directory that does not exist (e.g. tutorials in case of a reference-only archive).
While this is a rather special use case, the action basically supports it (except for this error) and we use it to have a unified documentation setup for our packages by generating the doccarchive for each target and then merging them together. For multi-target packages this works fine, but for single-target packages it currently fails.

This fixes the source of this error. Alongside, I've also added a small check to not create directories that are not needed - even when merging multiple archives. With that check, merging two reference-only archives will not create an empty tutorials directory.

Dependencies

None

Testing

Steps:

1. Create a single doccarchive (with only tutorials or references)
2. docs merge /path/to/archive.doccarchive --output-path ./combined.doccarchive

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[heckj]

@swift-ci please test