Fix typos and closing javadoc tag in RemoteCommand
Remove nullOrEmpty check for setSessionID()

in AdHocCommandData.

This check was added in 3a37b71e193729a9714a9793fb444be513b774fc, but a
session ID can be null, if it's the first invocation. This prevented
RemoteCommand.execute() from executing, since RemoteCommand's sessionID
member will be null at the first invocation.