[ntuple] rearrange code to avoid a compiler warning

[silverweed]

Fix a warning in RNTupleSerializer

Checklist:

• [x] tested changes locally
• [ ] updated the docs (if necessary)

[github-actions[bot]]

Test Results

    17 files      17 suites   3d 5h 16m 24s :stopwatch:  2 714 tests  2 713 :white_check_mark: 0 :zzz: 1 :x: 43 528 runs  43 527 :white_check_mark: 0 :zzz: 1 :x:

For more details on these failures, see this check.

Results for commit 09f8b94c.

:recycle: This comment has been updated with latest results.

[dpiparo]

Closed and opened to check the new container images, where TF 2.4 is installed, and not 2.5 that does not work on ARM64.