[TTree] when merging, ignore trees without branches

[ferdymercury]

This Pull request:

Alternative to #17650 as suggested by pcanal

Changes or fixes:

Fixes #14558
Fixes #12510

[github-actions]

Test Results

    18 files      18 suites   4d 6h 29m 39s ⏱️
 2 719 tests  2 719 ✅ 0 💤 0 ❌
47 253 runs  47 253 ✅ 0 💤 0 ❌

Results for commit 7f6cce8.

♻️ This comment has been updated with latest results.

[pcanal]

Part of my confusion is that #12510 is indeed about a file with branches and no entries.
In this case, the problem is not that there is missing branches or missing entries but rather that the first entry (number 0) in the output file is corrupted (has the wrong values).

With the files https://github.com/root-project/root/files/11333378/dummyFiles.zip, we get:

$ root.exe -b -l -q dummyFiles/dummyFile0.root -e 'dummyTree->Scan("*","","",2,0);'

Attaching file dummyFiles/dummyFile0.root as _file0...
(TFile *) 0x22986b0
************************************
*    Row   * index.ind * dummyVari *
************************************
*        0 *         0 * 0.2612867 *
*        1 *         1 * 0.7484587 *
************************************

$ hadd -f dummyFile10.root dummyFiles/dummyFile1.root dummyFiles/dummyFile0.root 
Info in <hadd>: target file: dummyFile10.root
Info in <hadd>: compression setting for all output: 101
hadd Source file 1: dummyFiles/dummyFile1.root
hadd Source file 2: dummyFiles/dummyFile0.root
hadd Target path: dummyFile10.root:/

$ root.exe -b -l -q dummyFile10.root -e 'dummyTree->Scan("*","","",2,0);'

Attaching file dummyFile10.root as _file0...
(TFile *) 0x2f39310
************************************
*    Row   * index.ind * dummyVari *
************************************
*        0 *         0 * 2.13e-306 *
*        1 *         1 * 0.7484587 *
************************************

I.e. Row 0 is wrong.

Nonetheless, it might (or might not) be fixed by this PR, in particular in my last proposal the part:

     if (newtree && nentries == 0)
        continue; // If we already have the structure and we have no entry, save time and skip

i.e. cloning the structure even if there is no entries, might solve the issue.

[pcanal]

Thanks.