`RDF::Describe` returns an incorrect file count

[enirolf]

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Description

Calling GetNFiles() on the object returned by Define gives an incorrect value. The underlying issue is the fact that the RDFDescription gets constructed without this number and thus fFileCount does not get properly initialized. It should be fixable by adding it as an argument in this line: https://github.com/root-project/root/blob/f53eb1b4f7c5df1350b0a2756695c8e951e8a34f/tree/dataframe/src/RInterfaceBase.cxx#L286 Additionally, perhaps the constructor without this arguments should be removed altogether to avoid similar bugs in the future.

Reproducer

ROOT::RDataFrame df(1);
df.Describe.GetNFiles();

ROOT version

v6.30 and up

Installation method

any

Operating system

any

Additional context

No response