don't crash on an extension with an empty declaration

[QuietMisdreavus]

Bug/issue #, if applicable: rdar://110683602

Summary

In current development builds of the Swift compiler, some extension block symbols are emitted with an empty declarationFragments array. This causes the extension block symbol converter to crash with an out-of-bounds access when it tries to strip away the extension SomeType part of the declaration to find its where clause. This PR fixes this logic by guarding against the length of the array as well as its existence.

Dependencies

None

Testing

I haven't isolated how to properly reproduce this (other than "export-import Foundation in a project"), so the test is restricted to the sample data i added in this branch.

Steps:

1. swift run docc convert 'Tests/SwiftDocCTests/Test Bundles/ModuleWithEmptyDeclarationFragments.docc'
2. Ensure that docc does not crash when processing the included extension symbol graph.

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
• [ n/a ] Updated documentation if necessary

[QuietMisdreavus]

@swift-ci Please test

[d-ronnqvist]

LGTM

[d-ronnqvist]

Did I understand correctly that we don't create a page for the empty extension?

[QuietMisdreavus]

Correct. The empty extension has no child symbols to curate, so it is not added to the final documentation.

[theMomax]

LGTM, thanks!