xAODEgamma : clang- tidy

Most of the changes come from

• https://clang.llvm.org/extra/clang-tidy/checks/readability-const-return-type.html i.e unneeded const in return arguements.
• https://clang.llvm.org/extra/clang-tidy/checks/readability-container-size-empty.html : Use empty, rather than size()==0
• https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html : Use nullptr