🐛 Enable 0.x with negative margins @ v4 (#33593)

* 🐛 Enable `0.x` with negative margins @ v4

I set it as below, but no negative margin was added.
`$spacers: (0\.5: ($spacer * .125));`

After investigating, it seems that `0 \ .5` is also evaluated as 0.
So I changed it to string comparison.

HTML example: `<img class="mt-n0.5">`

* 💚 Clear CI errors

scss/utilities/_spacing.scss

@@ -30,7 +30,7 @@
 
     // Negative margins (e.g., where `.mb-n1` is negative version of `.mb-1`)
     @each $size, $length in $spacers {
-      @if $size != 0 {
+      @if "#{$size}" != "0" {
         .m#{$infix}-n#{$size} { margin: -$length !important; }
         .mt#{$infix}-n#{$size},
         .my#{$infix}-n#{$size} {