Entries appended without a new line

[serialfuzzer]

I have two files:
first.txt (Doesn't have an empty line in the end)

1
2
3

second.txt

1
2
3
4
5

cat second.txt | anew first.txt

cat first.txt

The solution might be to add a new line if previous character wasn't a new line before appending new entries to the file.