[theme] Fix spacing string arguments

[GuilleDF]

Bug breaking change behavior

• [theme] Custom spacing functions need to handle all the arguments, e.g. auto was incorrectly omitted. This restores the previous behavior.

---

This resolves #21278. The change gives more control to the developers that choose to provide a function to the unit.

When passing a function to createSpacing, only number arguments were passed to the function, while strings were not.

Before:

const spacing = createSpacing((factor) => `${0.25 * factor}rem`);
expect(spacing(1, 'auto', 2, 3)).to.equal('0.25rem auto 0.5rem 0.75rem');

After:
With this PR we pass the strings to the function and expect the function to handle them:

spacing = createSpacing((factor) =>
  typeof factor === 'string' ? factor : `${0.25 * factor}rem`,
);
expect(spacing(1, 'auto', 2, 3)).to.equal('0.25rem auto 0.5rem 0.75rem');

• I have followed (at least) the PR section of the contributing guide.

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against cbc257c

[oliviertassinari]

@GuilleDF Thanks!