title: SASS/SCSS remove hyphen(-) if map has key called null?
body: <p>I want to remove <strong>hyphen(-)</strong> if map <strong>$spacers</strong> has key called <strong>null</strong>.
  means it return only the <strong>className</strong> is this case it called <strong>m</strong> without <strong>hyphen(-)</strong>.</p>
  <pre><code>$spacers: (
    0: 0,
    1: 1px,
    2: 2px,
    null: 3px,
    4: 4px,
    8: 8px
  ) !default;
  
  @each $prop, $abbrev in (margin: m) {
     @each $size, $length in $spacers {
        .#{$abbrev}-#{$size} {
           #{$prop}: $length !important;
        }
     }
  }
  </code></pre>
  <p><strong>the result that I get after compiling :</strong></p>
  <pre><code>.m-0 {
    margin: 0 !important;
  }
  
  .m-1 {
    margin: 1px !important;
  }
  
  .m-2 {
    margin: 2px !important;
  }
  
  // hyphen(-) shouldn't be here
  .m- {
    margin: 3px !important;
  }
  
  .m-4 {
    margin: 4px !important;
  }
  
  .m-8 {
    margin: 8px !important;
  }
  </code></pre>