And here is an awk solution that will strip blank lines, not only null lines but also empty lines containing only white space (spaces and/or tabs). It is selecting all lines where the number of fields (NF) on a line are greater than zero:

awk 'NF>0' in_file > out_file

And one awk solution to strip just the completely null lines:

awk 'length==0' in_file > out_file

And an awk solution to add a blank line after each line:

awk '{print $0;print ""}' in_file > out_file

Or to insert blank lines only where needed, considering that there might already be some blank lines:

awk '{if (NF>0&&prev>0) print "";print $0;prev=NF}' inf > outf