i have cell A1 = 1,0,0,1 cell B1=,1,,-1

I need a formula that would be the equivalent of =update(A1,B1) The numbers between the commas can be any number. I need B1 to replace data in A1 where there is a number.

output: 1,1,0,-1

Note: A1 and B1 will always be in that format, 4 numbers separated by commas.