Nginx location - auto-substitute another image in the absence of the requested image?

Question

Nginx

There is a set of pictures located on the server at:

/var/www/site.com/web/images/storage/icons/ИКАНОКА.png

It is simply requested from the browser:
https://site.com/images/storage/icons/TEST1.png

You need to check for the presence of icons by the file name:
1. First, the requested “TEST1.png”:
https://site.com/images/storage/icons/TEST1.png
if it exists → give it out

2. Then it’s the same but “.gif”
https://site.com/images/storage/icons/TEST1.gif
if it exists → give it

out 3. If there is no p. 1, no item 2 - issue a stub:
/images/storage/icons/_NULL.png

This is not how it works:

location ^~ /images/storage/icons/([a-zA-Z0-9]{1,9})\.(gif|png)$ {
    try_files $uri /images/storage/icons/$1.png /images/storage/icons/$1.gif /images/storage/icons/_NULL.gif =404;
}

so too:

location ^~ /images/storage/icons/([a-zA-Z0-9]{1,9})\.(gif|png)$ {
    try_files $uri /var/www/site.com/web/images/storage/icons/$1.png /var/www/site.com/web/images/storage/icons/$1.gif /var/www/site.com/web/images/storage/icons/_NULL.gif =404;
}

Tell me, pliz, how to set up the check correctly?