perl -pe '$_=/\S/?do{chop;$_.",".<>}:""'

##</code><code>##

286,0 sauoq@grover:~$ perl -l054p012e'($\,$a)=($a||$\,$b||$/);$b=$\' < test.txt
a,b
,c
d,287,0 sauoq@grover:~$ cat test.txt
a
b

c
d
288,0 sauoq@grover:~$