xAOD . Clang-tidy : nullptr,emplace_back,use empty

xAOD . Clang-tidy :

• use nullptr,
• qualify auto
• use emplace_back
• use empty
• make_unique vs reset from plain ptr.
• Simplify boolean expressions.
• using vs typedef (in case anyone wonders this is the "using" case allowed in our styleguide "This rule does not apply when using is used to define a new name for a type (similarly to typedef)" ) and seems to be preferred for "modern" C++