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.

Cell C1 output: `1,1,0,-1`

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