`null:` as a target file writes an actual file

``` magick ~/Downloads/a/pexels-andrea-yurko-364018-982585.jpg -format 'Image #%p named \"%f\" is a %m\n with size \"%wx%h\"' -identify null: ``` Doesn't produce a file. ``` cargo run -- ~/Downloads/a/pexels-andrea-yurko-364018-982585.jpg -format 'Image #%p named \"%f\" is a %m\n with size \"%wx%h\"' -identify null: ``` produces ``` -rw-r--r-- 1 hanneskaeufler staff 352343 Oct 2 23:18 null: ```