Fixes empty suites array still adding a ClientSuite

Also tests against the suites argument instead if it exists, which in conjunction with https://github.com/theintern/intern/pull/269 will not add a ClientSuite when an empty suites argument is specified.

See https://github.com/theintern/intern/pull/241#issuecomment-55562168 and https://github.com/theintern/intern/commit/4a6512b408304d83e30d8dadf01055823f36d12c#commitcomment-7780521.