How do I print line numbers but reset line counter at empty line?

I have a file.txt containing:

this is the first
second line
not last line

fourth but first
second in list
seventh in file
seventh with nl

Normally I would just cat and pipe | it into nl like so:

$> cat file.txt | nl
1  this is the first
2  second line
3  not last line

4  fourth but first
5  second in list
6  seventh in file
7  seventh with nl

But I need the line numbers to reset when it encounters an empty line like so:

$> alias_or_function file.txt
1  this is the first
2  second line
3  not last line

1  fourth but first
2  second in list
3  seventh in file
4  seventh with nl

How could I do this using a quick function or alias in my ~/.zshrc?