[tutorials] Return void instead of TCanvas* in histogram tutorials

[siliataider]

This Pull request:

Update the histogram tutorials to return void instead of a TCanvas*, which is also the convention used in other ROOT tutorials.

This is required for testing in the Python wheels workflow #19600 since running root tutorial.C in this context returns code 255 , causing pytest to flag the test as failed.

See failure message:

FAILED test/wheels/test_tutorials.py::test_cpp_tutorial[hist004_TH1_labels.C] - subprocess.CalledProcessError: Command '['/opt/hostedtoolcache/Python/3.11.14/x64/bin/root', '-b', '-q', '/opt/hostedtoolcache/Python/3.11.14/x64/lib/python3.11/site-packages/ROOT/tutorials/hist/hist004_TH1_labels.C']' returned non-zero exit status 255.

Checklist:

• tested changes locally

[hahnjo]

Tutorials are run as part of the CI, so I would argue the PR should not be marked [skip-ci] and only discover problems after the merge (it happened in the past)

[linev]

Also please start CI - some tutorials used for testing, which rely on the fact that non-zero value returned by such tutorial.

[linev]

See this list: https://github.com/root-project/root/blob/master/tutorials/CMakeLists.txt#L547-L562

[linev]

Now looks better - but let see CI first

[github-actions]

Test Results

    19 files      19 suites   4d 1h 7m 56s ⏱️
 3 707 tests  3 704 ✅ 0 💤 3 ❌
68 722 runs  68 719 ✅ 0 💤 3 ❌

For more details on these failures, see this check.

Results for commit 4d5d0f9.

♻️ This comment has been updated with latest results.

[linev]

Now looks ok. Please wait for CI