fix warning "Use of uninitialized value in subroutine entry"

Clears up the following warning from Data::Dumper when object properties end up undefined, in this case due to undef Maxdepth package variable.
```
Use of uninitialized value in subroutine entry at /software/perl/lib/perl5/x86_64-linux-thread-multi/Data/Dumper.pm line 224.
```