Make $differing-right-padding work without $padding

[ChristianPeters]

@function left-and-right-sum($value, $differing-right-value)
  @if type-of($differing-right-value) == number
    @if comparable($value, $differing-right-value)
      @return $value + $differing-right-value
    @else
      @warn "It is not possible to compute the sum of #{$value} and #{$differing-right-value}. Please make them comparable. Meanwhile, the result is set to #{$value * 2}."
  @return $value * 2

This function issues just a warning when $value is not comparable with $differing-right-value but it is okay if $value is not a number to just return $differing-right-value

[ChristianPeters]

Hmm shouldn't be an issue because the default is 0 which should be comparable?

Maybe it didn't work because it's a grid-container?