Javadoc says use empty string not null for no agreed sub-protocol

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1460648 13f79535-47bb-0310-9956-ffa450edef68
Partial fox for https://issues.apache.org/bugzilla/show_bug.cgi?id=54734
Add new throws
Based on a patch by Nick Williams

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1460669 13f79535-47bb-0310-9956-ffa450edef68