[Stack, system] Apply correct responsive styles if any custom breakpoints are provided

[ZeeshanTamboli]

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

Fixes #32214

---

First of all, a very intriguing bug.

See the following:

Particularly interesting, since the issue reporter was having a small custom breakpoint value and for other breakpoints properties it was not causing any issue.

And if the input of direction or spacing is a string in Stack (here it's direction="column"), this particular line will evaluate to true due to it being a function as shown in the above image.

The String object has small method in it's prototype. See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/small.

It's wrong to access the property if the input is a string. So first we check that it's of type object, then only we create a custom base.
Same applies to resolveBreakpointValues function. So, it's also a regression from #29300.

Before: https://codesandbox.io/s/peaceful-https-6iewi9?file=/src/Demo.tsx
Now: https://codesandbox.io/s/jolly-volhard-247y2y?file=/src/Demo.tsx

[mui-bot]

Details of bundle changes

Generated by 🚫 dangerJS against a46cd9f

[ZeeshanTamboli]

Also, not complaining, but just wanted to say that TypeScript would have prevented this type of issue. 😁

[mnajdova]

This is what I had in mind, sorry if it was not clear.

[mnajdova]

Logic looks good 👍