fix: do not generate invalid xy-cell when breakpoint is not found

[ncoden]

For the following code:

.test {
  @include xy-cell(12, $breakpoint: small);
  @include xy-cell(12, $breakpoint: xxx-invalid-breakpoint-xxx);
}

Generate the following code:

.test {
  width: calc(100% - 1.25rem);
  margin-right: 0.625rem;
  margin-left: 0.625rem;
}

And gives this warning:

WARNING: xy-cell: no gutters were found in `$gutters` for "$breakpoint: xxx-invalid-breakpoint-xxx", cell was not generated`
Backtrace:
	../foundation-sites/scss/xy-grid/_cell.scss:142, in mixin `xy-cell`
	...

Not: a refactor of breakpoints seems necessary to ensure we have this behavior across the whole framework.

[ncoden]

IamManchanda self-assigned this 23 hours ago

@IamManchanda Do you want to review this ?

[ncoden]

poke @IamManchanda