fix(storybook): dont fail if targets dont exist

[mandarini]

Current Behavior

If there is no targets object in project.json then generator fails.

Expected Behavior

If there is no targets object in project.json generator should not fail.

Related Issue(s)

Fixes #18725

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
nx-dev	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 21, 2023 2:59pm

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✅ Successfully ran 8 targets

• linux nx affected --target=e2e --base=2638bb0db2839051c7b661a56199dccf5ee96086 --head=76464d16a6209b8a60a8a2067c2942594ced06e8 --parallel=1
• linux nx affected --target=test --base=2638bb0db2839051c7b661a56199dccf5ee96086 --head=76464d16a6209b8a60a8a2067c2942594ced06e8 --parallel=1
• linux nx affected --target=build --base=2638bb0db2839051c7b661a56199dccf5ee96086 --head=76464d16a6209b8a60a8a2067c2942594ced06e8 --parallel=3
• linux nx run-many -t check-imports check-commit check-lock-files check-codeowners documentation --parallel=1 --no-dte
• linux nx affected --target=lint --base=2638bb0db2839051c7b661a56199dccf5ee96086 --head=76464d16a6209b8a60a8a2067c2942594ced06e8 --parallel=3
• nx-cloud record -- nx format:check --base=2638bb0db2839051c7b661a56199dccf5ee96086 --head=76464d16a6209b8a60a8a2067c2942594ced06e8
• linux nx documentation --no-dte
• macos nx affected -t e2e-macos --parallel=1 --base=2638bb0db2839051c7b661a56199dccf5ee96086 --head=76464d16a6209b8a60a8a2067c2942594ced06e8

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[github-actions]

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