Add stub for ttreeindex to supress warning

[tamasgal]

When a TTreeIndex is present, UnROOT currently shows a warning (which make some users panic):

  julia> foff = ROOTFile("/Users/tamasgal/Data/KM3NeT_00000133_00015285.mc.gsg_astro-neutrinos_merged.sirene.jterbr.jpp
  muon_aashower_static.offline.v9.1.root")
  ROOTFile with 4 entries and 44 streamers.
  /Users/tamasgal/Data/KM3NeT_00000133_00015285.mc.gsg_astro-neutrinos_merged.sirene.jterbr.jppmuon_aashower_static.off
  line.v9.1.root
  ┌ Warning: Could not get streamer for 'TTreeIndex' (TKey: UnROOT.TKey32(11058, 4, 57430, 0x76a11d5d, 53, 1,
  219387122, 100, "TTree", "KM3NET_SUMMARYSLICE", ""))
  └ @ UnROOT ~/Dev/UnROOT.jl/src/streamers.jl:255
  ├─ KM3NET_SUMMARYSLICE (TTree)
  │  └─ "KM3NET_SUMMARYSLICE"
  ├─ E (TTree)
  │  └─ "Evt"
  ├─ Head (Head)
  └─ META (TDirectory)
     ├─ JMeta (TNamed)
     ├─ JMeta (TNamed)
     ├─ JMeta (TNamed)
 ...
....
...

I added a stub to make it disappear 😉 Maybe we should use this TTreeIndex at some point

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.82%. Comparing base (bcb74a7) to head (20b48e5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bootstrap.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #416      +/-   ##
==========================================
- Coverage   86.85%   86.82%   -0.03%     
==========================================
  Files          24       24              
  Lines        3370     3371       +1     
==========================================
  Hits         2927     2927              
- Misses        443      444       +1     

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