Allow false in the sx TypeScript types

[bartlangelaan]

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

The following snippet is from the documentation:

<div
  sx={[
    { color: 'red' },
    selected && { fontWeight: 'bold' },
    disabled ? (theme) => ({ opacity: theme.state.disabledOpacity }) : { opacity: 1 },
  ]}
/>

However, this generates a TypeScript error that boolean is not allowed in the sx prop.

This PR allows for false values to be in the SxProp array, like it is allowed in @mui/system.

[siriwatknp]

Can you add another test to verify this?

let selected: boolean | undefined;
const sx4: SxProp = [
  selected && { color: 'red' },
]