[ntuple] Merger: fix bugs in GenerateZeroPagesForColumns

[silverweed]

• nElements was used to grow each zero page instead of nElementsPerPage
• only the last zero page was added to the page groups

This bug manifested as a crash when Union-merging big-enough RNTuples, so that GenerateZeroPagesForColumns actually generates more than 1 page.

This fix will be backported to 6.36.

cc @amete

Checklist:

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

[github-actions]

Test Results

    20 files      20 suites   3d 7h 19m 35s ⏱️
 3 050 tests  3 048 ✅   0 💤 2 ❌
59 400 runs  59 261 ✅ 137 💤 2 ❌

For more details on these failures, see this check.

Results for commit eae9482.

[hahnjo]

Thanks!

[jblomer]

Thanks!