GH-3647: Allow 'uploading all' when loading into the default graph

[ThomasThelen]

GitHub issue resolved #3647

Pull request Description:

This PR resolves an issue where loading all files at once without setting a named graph would be blocked. The core change is here, where we make the statement that empty graph names are valid names.

The change here fixes a small issue that I ran acroess where the original code short circuits when this.validateGraphName is False, leaving the potential to miss the error from this.validateFiles. The logic leads to only one of two errors being shown. This is likely seen in the screenshot in the attached issue where, after clicking Upload All with no files, there should have been a second error shown.

As for why this wasn't affecting single file uploads... It doesn't look like the named graph name is getting gated (it gets watched by this but submission isn't rejected) when going that route.

Example after this PR:

Screen.Recording.2026-07-28.at.11.56.34.PM.mov

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• Tests are included.
• Documentation change and updates are provided for the Apache Jena website
• Commits have been squashed to remove intermediate development commit messages.
• Key commit messages start with the issue number (GH-xxxx)

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See the Apache Jena "Contributing" guide.