[system] spacing not giving warning that theme.spacing array cannot be combined with non integer value

[mnajdova]

• I have searched the issues of this repository and believe that this is not a duplicate.

Summary 💡

When there is array of values specified for the spacing inside the theme, the spacing system function returns unexpected value, without adding any warning that this isn't supported.

We could probably fix this by making this test pass:

 it('should error dev that array values for the theme.spacing cannot be combined with non integer values for the system', () => {
    let output;

    expect(() => {
      output = spacing({
        theme: {
          spacing: [1, 2, 3, 4, 5, 6],
        },
        p: 0.5,
      });
    }).toErrorDev(
      'Material-UI: The `theme.spacing` array type cannot be combined with non integer values.\n' +
        'You should either use an integer value that can be used as index, or define the `theme.spacing` as a number.',
    );

    expect(output).to.deep.equal({ padding: undefined });
});

Examples 🌈

This is one example of non valid usage

 const output = spacing({
   theme: {
     spacing: [1, 2, 3, 4, 5, 6],
   },
   p: 0.5,
 });

Motivation 🔦

This will explicitly ensure that we are thinking about all invalid cases that can be used, and can guide the developers towards the correct approach.

First reported on #23053 (comment)