leave cell blank until formula has all info to complete task, I should know this but, blonde showing

Allthingstiny 

If 'X' and 'Y' were defined names given to the two columns, the specific test for presence of numbers is the function ISNUMBER.

= IF(ISNUMBER(X)*ISNUMBER(Y), X*Y, "")

With 365 insider, I can play all sorts of games to upset a traditional spreadsheet author.  For example, given 'data' that includes both 'X' and 'Y',

= BYROW(Data, LAMBDA(d,
    IF(AND(ISNUMBER(d)),PRODUCT(d),"")
  ))

 will return the entire column of results.

"Blond showing"  I hope this does not make you grab the peroxide bottle!