Add Stipulation That Specific Cell Must Not Be Blank To Existing Code

I have a code now that in excel, when the button is clicked, will put file into a new email and format the email.

What I have been trying to do, is make it so that if Cell C10 is blank, you get a prompt telling you it must be filled out before you can proceed with the submission.

Here is the code now:

Sub SendTimeCard()
On Error GoTo err_handler
Dim OutApp As Object
Dim OutMail As Object

Dim subject As String
Dim body As String
' Be sure the workbook is saved first
ThisWorkbook.Save

[Code] ........