feat(core): provide a hint when project.json has empty targets

[jaysoo]

With Project Crystal, new projects are generated with empty targets in project.json. This is confusing for users that expect targets to be defined in project.json or nx.json.

This PR ensures that whenever the targets in project.json are empty, we add a hint for the user:

{
  "// targets": "to see all targets run: nx show project <name> --web`",
  "targets" {}
}

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Name	Status	Preview	Updated (UTC)
nx-dev	⬜️ Ignored (Inspect)	Visit Preview	Feb 29, 2024 3:07am

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✅ Successfully ran 6 targets

• nx affected --target=e2e --base=f6a183c4f7cea9a4e336f5fc6d3728a3a8f3820e --head=a9b1fe82ed0717b3871751dd6edb82fb12f3c968 --parallel=1
• nx run-many -t check-imports check-commit check-lock-files check-codeowners documentation --parallel=1 --no-dte
• nx affected --targets=lint,test,build --base=f6a183c4f7cea9a4e336f5fc6d3728a3a8f3820e --head=a9b1fe82ed0717b3871751dd6edb82fb12f3c968 --parallel=3
• nx-cloud record -- nx format:check --base=f6a183c4f7cea9a4e336f5fc6d3728a3a8f3820e --head=a9b1fe82ed0717b3871751dd6edb82fb12f3c968
• nx documentation --no-dte
• nx affected -t e2e-macos --parallel=1 --base=f6a183c4f7cea9a4e336f5fc6d3728a3a8f3820e --head=a9b1fe82ed0717b3871751dd6edb82fb12f3c968

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