Are you asking how to send emails or how you would condition the sending of an email?
If it's the latter....
I would suggest you perform a Select Count(*) from ... on the criteria you are using to create the text file. Store the result of the count in a Global Variable. In a separate task (ActiveX script or whatever method you use to send emails), check if the count returns zero and then send the email.
You can send emails by using xp_sendmail, or by using an SMTP email process, or the Email task within DTS.
Hope this helps.