I had an alternative 4 node solution
I preferred it to a solution involving “Sorter”, simply because Sorter is going to sort ALL of your rows, so you would need to take care to ensure you don’t reorder your data where you don’t want to.
I was disappointed though at not being able to find a way to improve on @ArjenEX’s Column Expressions solution 
and so…
… here is my trusty friend String Manipulation with a single node solution, using a java hack… but only because otherwise @gonhaddock will read this and be disappointed 
string(
java.util.Arrays.stream($Item List$.split(",")).sorted()
.collect(java.util.stream.Collectors.joining(","))
)
It was @ArjenEX 's use of arraySort() in Column Expressions that made me think of it. Yes it’s another undocumented “java hack”, but I find it fun 
Sorting comma separated list.knwf (18.2 KB)
linking to Fun with String Manipulation