Fix eunit_surefire crash with rebar eunit tests=...

This fixes the following crash:

> =ERROR REPORT==== 20-Jun-2014::16:35:38 ===
> Error in process <0.40.0> with exit value:
{function_clause,[{eunit_surefire,escape_suitename,[undefined,[]],[{file,"eunit_surefire.erl"},{line,416}]},{eunit_surefire,write_report,2,[{file,"eunit_surefire.erl"},{line,258}]},{lists,foreach,2,[{file,"lists.erl"},{line,1336}]},{...