[tree] public method should be safe against fImpl = null

as is the case with function GetReadStatus

This prevents the crash on line 35 (stack trace by amadio on https://its.cern.ch/jira/browse/ROOT-11006):

Author: ferdymercury

tree/treeplayer/inc/TTreeReaderArray.h

@@ -32,7 +32,7 @@ Base class of TTreeReaderArray.
                            TDictionary* dict):
          TTreeReaderValueBase(reader, branchname, dict) {}
 
-      std::size_t GetSize() const { return fImpl->GetSize(GetProxy()); }
+      std::size_t GetSize() const { return fImpl ? fImpl->GetSize(GetProxy()) : 0; }
       bool IsEmpty() const { return !GetSize(); }
 
       EReadStatus GetReadStatus() const override { return fImpl ? fImpl->fReadStatus : kReadError; }