Changes in vertexing-fwd task

[sarahherrmann]

• Corrects a bug when the number of collisions associated to an ambiguous track is 0
• New way to access to collisions, without the table aod::MatchedBCCollisionsSparse that did not take into account the fact that a BC could be associated to 2 collisions

[alibuild]

Error while checking build/O2Physics/o2 for fe0c39d at 2022-07-14 16:51:

## sw/BUILD/O2Physics-latest/log
/sw/SOURCES/O2Physics/995-slc7_x86-64/0/PWGMM/Mult/Tasks/vertexing-fwd.cxx:211:73: error: format '%lld' expects argument of type 'long long int', but argument 3 has type 'int64_t' {aka 'long int'} [-Werror=format=]
ninja: build stopped: subcommand failed.

Full log here.