If your welcome emails are blank, please do the following:
Go to "Setup>>Settings>>Emails" , select 'Welcome E-mails' from the drop-down. Then you will see your various welcome e-mail templates, click on the welcome e-mail template for editing and make sure that you have filled both html and text sections.
Also, make sure that there isn't any unwanted tags like span present in the welcome e-mail html ( you can see the source code by clicking on the 'Source Edit' button in the template box ).