$ cat tri.txt
aaa|1234||2bc||rahul|tamilnadu
bba|2234||b4c||bajaj|tamilnadu

$ awk 'BEGIN{OFS=FS="|"}{ for(k=0;k<=NF;k++)
if ( $k == "" )
$k="NULL"
print $0}' tri.txt

aaa|1234|NULL|2bc|NULL|rahul|tamilnadu
bba|2234|NULL|b4c|NULL|bajaj|tamilnadu