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. 

more examples: 

A1 = `1,0,0,1` cell B1=`,1,,-1` --> `1,1,0,-1`

A1 = `10,0,0,1` cell B1=`,1,,-1` --> `10,1,0,-1`

A1 = `10,0,0,01` cell B1=`,1,,-1` --> `10,1,0,-1`

A1 = `10,0,-1,0` cell B1=`,1,,-1` --> `10,1,-1,-1`

A1 = `10,1,0,1` cell B1=`,0,,-1` --> `10,0,0,-1`

A1 = `10,0,0,01` cell B1=`-1,1,,` --> `-1,1,0,01`

General:  
A1 = `a,b,c,d` B1 = `e,f,g,h` --> make C1 by replacing `a` with `e`, `b` with `f`... etc, if `e,f,g,h` are not blanks.