grid: checks for $grid-columns > 0

[zalog]

This PR adds:

• a check for $grid-columns > 0
• permits us to use to assign $grid-columns: 0; and removing all .col-[number] when needed

[MartijnCuppens]

We already have the $enable-grid-classes variable, which covers this.

[mdo]

Yeah, and I favor the explicit option of $enable-grid-classes vs something implicit based on a variable's value. Seems safe to close?

[zalog]

Sorry @mdo, @MartijnCuppens and I, have some slack dm's on this subject and I forgot to log a follow up here.

Here is a use case for a grid version, a minimal one:

$enable-grid-classes: true; // option as is default
$grid-columns: 0; // covered by this PR
$grid-row-columns: 0; // covered by #30606
$grid-breakpoints: (); // covered by #30609

So after this, our grid will look like:
.row {...}
.row > * {...}
.col {...}
.row-cols-auto > * {...}
.col-auto {...}

So no .col-* and .offset-*. This config, but with some $grid-breakpoints of course :) can be a valid choice if we don't need cols number and we just want equals one or auto.

What do you think?

I think that it's a small change for a big opportunity of improving css filesize when our users doesn't need the whole grid :)

[MartijnCuppens]

It's an edge case, but since it only adds a single if() test and prevents our codebase from crashing if $grid-columns: 0;, this is worth the change.