Fix usage of the MeanVertex in PVertexer

[shahor02]

No description provided.

[alibuild]

Error while checking build/O2/fullCI_slc9 for c8c615d at 2026-06-30 19:31:

++ cp /sw/slc9_x86-64/O2/slc9_x86-64-slc9_x86-64-local12/compile_commands.json .
+++ python3 -c 'import json, os; print(os.path.commonpath([x["file"] for x in json.loads(open("compile_commands.json").read()) if "sw/BUILD" not in x["file"] and "G__" not in x["file"] and x["file"].endswith(".cxx")]))'
++ O2_SRC=/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0
++ [[ -e /sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/CMakeLists.txt ]]

Full log here.