ROB: Raise PdfReadError when missing /Root in trailer.

[BertrandBordage]

Fixes #2806.

When running the same code as described in #2806 with the same PDF, now this happens:

>>> list(reader.pages)
Object 493 0 not defined.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/pypdf/pypdf/_page.py", line 2356, in __len__
    return self.length_function()
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/pypdf/pypdf/_doc_common.py", line 352, in get_num_pages
    self._flatten()
  File "/pypdf/pypdf/_doc_common.py", line 1100, in _flatten
    catalog = self.root_object
              ^^^^^^^^^^^^^^^^
  File "/pypdf/pypdf/_reader.py", line 195, in root_object
    raise PdfReadError('Cannot find "/Root" key in trailer')
pypdf.errors.PdfReadError: Cannot find "/Root" key in trailer

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.86%. Comparing base (d2d520b) to head (9e73fb9).
Report is 78 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2808   +/-   ##
=======================================
  Coverage   95.85%   95.86%           
=======================================
  Files          51       51           
  Lines        8573     8576    +3     
  Branches     1695     1696    +1     
=======================================
+ Hits         8218     8221    +3     
  Misses        212      212           
  Partials      143      143           

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