[ntuple] Properly set the NTupleName in sources created from anchors

[silverweed]

The previous code would always set the name to the empty string as the descriptor builder was not initialized yet by that point.

Checklist:

• tested changes locally
• updated the docs (if necessary)

[enirolf]

Nice, thank you!

[github-actions]

Test Results

    22 files      22 suites   3d 23h 45m 21s ⏱️
 3 683 tests  3 683 ✅ 0 💤 0 ❌
79 131 runs  79 131 ✅ 0 💤 0 ❌

Results for commit 572a0ba.