Add missing default value for regexps in reltool

It caused a function clause in lists:sort/1:

  reltool_server_SUITE.erl(2512): <ERROR>
  Not matching actual result was:
   {error,
       {function_clause,
           [{lists,sort,
                [[{regexp,"^priv",
                      {re_pattern,0,1,
                          <<69,82,67,80,64,0,0,0,16,8,0,0,0,0,0,0,0,0,0,0,0,
                            0,0,0,48,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
                            0,0,0,93,0,12,25,27,112,27,114,27,105,27,118,84,
                            0,12,0>>}}|
                  undefined]],
                [{file,"lists.erl"},{line,465}]},
            {reltool_server,decode,2,[{file,"reltool_server.erl"},{line,1499}]},
            {reltool_server,decode,2,[{file,"reltool_server.erl"},{line,1363}]},
            {reltool_server,read_config,2,
                [{file,"reltool_server.erl"},{line,1335}]},
            {reltool_server,parse_options,4,
                [{file,"reltool_server.erl"},{line,224}]},
            {reltool_server,do_init,1,[{file,"reltool_server.erl"},{line,154}]},
            {reltool_server,init,1,[{file,"reltool_server.erl"},{line,133}]},
            {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,239}]}]}}
  Expected { ok , _ }