[docs][combobox] Fix creatable demo

[atomiks]

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

The demo from beta.4 had an issue where the creation dialog opened when clicking a non-creatable item. Refactored the logic to be less confusing here and fixes the bug

[pkg-pr-new]

vite-css-base-ui-example

pnpm add https://pkg.pr.new/mui/base-ui/@base-ui-components/react@3070

pnpm add https://pkg.pr.new/mui/base-ui/@base-ui-components/utils@3070

commit: f519caa

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui-components/react	0B(0.00%)	0B(0.00%)

Details of bundle changes

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