fix: use fallback gutter for fallback breakpoint in XY cell #11194

[ncoden]

Description

When no breakpoint is passed to xy-cell and a fallback "zero" breakpoint is used, use a fallback gutter as well if it is not found. This way, an implicit behavior stay implicit and a warning is thrown only if a breakpoint was explicitely given.

The following code:

$breakpoints: (
  xsmall: 0,
  small: 330px,
  medium: 640px,
  large: 1024px,
  xlarge: 1200px,
  xxlarge: 1440px
);

// (...)

.my-cell {
  @include xy-cell(12);
}

Now generate (instead of throwing a warning):

.test {
  width: 100%;
  margin-right: 0;
  margin-left: 0;
}

Closes #11194

Motivation and Context

Breakpoint could be added to $breakpoints while $grid-margin-gutters is left unchanged. A custom zero breakpoint like xsmall can be added before small. In this case, @include xy-cell(12); will fail because xy-cell will search for a gutter value for xsmall.

As xy-cell fallback to xsmall implicitly, I expect all values direved from it to also have a fallback, unless we require a zero-breakpoint value to be declared in all breakpoint-related values.

Screenshots (if appropriate):

Types of changes

• Documentation
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to change)

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[DanielRuf]

So 0 is always the fallback?

[ncoden]

@DanielRuf Yes. If no gutter is declared for the zero breakpoint, then we generate a cell with... no gutters.