IF Function To Return No Data As A Result

I've created a Database using excel which feeds data to a pivot table I created based on it. My objective is to create a new item in this pivot table (Pivot Table > Formulas > Calculated Item) which calculates the average data for the past 6 months.
The formula I used in order to get a result is this:

=IF(((May+June+July+August+September+October)/6)=0;;(May+June+July+August+September+October)/6)

and the problem is that the answer is 0 when there is no data for all 6 months (for example, there is a customer who hasn't baught anything during the past 6 months), the pivot table displays 0, instead of not showing this customer at all. My goal is for the IF function not to return any data if the result is 0, so that the pivot table doesn't show items with no data.
I also tried:
=IF(((May+June+July+August+September+October)/6)=0;"";(May+June+July+August+September+October)/6)

and also using ;" ";

but these formulas return #VALUE! in the pivot table, given that the Data is "SUM OF Sales" and "" is not a number.

How can I get the formula not to show anything at all?

In addition to this, I was wondering if it is possible to make this formula more automated, so that it calculates the average of the past 6 months, taking into account some type of reference month (rather than having to edit the formula each time a new month is analyzed).