Fix the null pointer dereference in the MuonTrackTruthTool reported in ATLASSQ-215 ATLASSQ-216

• Closes ATLASSQ-215 and ATLASSQ-216
• There are obvious bugs and null pointer dereferences.
• I'm not the author of the original code and was mentioned in the ATLASSQ-215/216 only because I did a cleanup and removed commented codes.
• The affected functions are not used anywhere in the Athena.
• Because of the last two points, it would be nice to have some input from experts/authors.

Andrii