[ntuple] add merger test for first input with 0 entries

[silverweed]

Adds a test to the merger where we merge 3 RNTuples with different schemas, the first 2 of which have 0 entries.
Verify that it behaves as expected (Filter and Strict fail, Union produces an output with the union of the schemas and non-deferred entries from the 3rd RNTuple)

Checklist:

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

[jblomer]

Nice! Perhaps extend the test to make the first two ntuples zero-sized.

[github-actions]

Test Results

    22 files      22 suites   3d 15h 1m 26s ⏱️
 3 705 tests  3 703 ✅ 0 💤 2 ❌
79 566 runs  79 564 ✅ 0 💤 2 ❌

For more details on these failures, see this check.

Results for commit 94ca0ae.