Creating a Month Over Month Column with Blank Data Present

Hello everyone,

I'm a bit lost on the right approach to achieve this. I'm attempting to create a Table or Matrix with a column showing the change in value from a user selectable month to another user selectable month. The kicker is that if there were no sales of an item for a specific month then I have no data to represent that. 

Long time viewer but first time poster. My attempt at an example table to show what I'm trying to achieve... : 

I found this code snippet which works if there are no blanks in the data. If there are blanks in the data then HASONVALUE comes up as True and it just takes the single month value instead of doing (0 -  value) or (value - 0).

How could I show a blank or gap in data as a 0? 

Month Change = 
    IF (
        hasonevalue ( 'Sheet1'[Period] ) , 
        SUMX (
            'Sheet1', 'Sheet1'[Sales]
        ),
        var mn = min ( 'Sheet1'[Period] )
        var mx = max ( 'Sheet1'[Period] )
        var mnsum = calculate ( SUM ('Sheet1'[Sales]) ,'Sheet1'[Period] = mn )
        var mxsum = calculate ( SUM ('Sheet1'[Sales]) ,'Sheet1'[Period] = mx )
           
        return 
            mxsum - mnsum
    )

Any help would be great! Thank you!