Fix problems with ART tests in master, r 22

From @fwinkl :

m_athenaTool.typeAndName() != "IPythia8Custom"

But typeAndName() is probably "IPythia8Custom/IPythia8Custom". However, I recommend you change the code and use an empty ("") ToolHandle by default. Then simply check via

if (not m_athenaTool.empty()) ...

The code is clearly trying to create a tool of type IPythia8Custom, which itself is an abstract interface. This cannot work.

Thanks also to @ewelina.

G