/// Set a CSS rule, but only if the key is defined in the values map.
///
/// @group helpers_logic
///
/// @parameter {Map}    $values - a Sass map
/// @parameter {String} $rule   - the CSS rule to set if `$key` is provided
/// @parameter {String} $key    - the key to look for in `$values`
///
/// @example scss - Usage
///   $font-style: (
///     letter-spacing: 0.05rem,
///     text-transform: uppercase
///   );
///
///   @include k-apply-if-set($font-style, font-weight,    'font-weight');
///   @include k-apply-if-set($font-style, letter-spacing, 'letter-spacing');
///   @include k-apply-if-set($font-style, text-transform, 'text-transform');
///
/// @example css - CSS output
///   letter-spacing: 0.05rem;
///   text-transform: uppercase;
@mixin k-apply-if-set($values, $rule, $key) {
  @if map-get($values, $key) {
    #{$rule}: map-get($values, $key);
  }
}