src/lib/dhcpsrv/client_class_def.cc
    - updated comment on empty expressions in ctor,
    - cleaned up whitespace

src/lib/dhcpsrv/parsers/client_class_def_parser.cc
    - ExpressionParser::build() - integrated use of Eval
    parsing in ExpressionParser
    - cleaned up whitespace

src/lib/dhcpsrv/tests/client_class_def_parser_unittest.cc
    Updated tests to verify that expressions are actually
    parsed and function.