Fix --customer-account-push flag when no domain is set in .env

[frandiox]

Related to #1992

A bit of a tricky issue:

• Right now, if you pass --customer-account-push__unstable and have no PUBLIC_STORE_DOMAIN in .env, it will think you're using mock.shop and skip the tunnel. This would be correct if we had all the variables at this point, but this is only considering local variables. The domain might be injected later from remote/admin variables.
• A value like PUBLIC_STORE_DOMAIN="" in .env overwrites remote values and uses mock.shop. We still need to disable tunnel in this case.

To 🎩 , ensure --customer-account-push__unstable has the expected behavior when:

• PUBLIC_STORE_DOMAIN is set to a real domain (e.g. hydrogen-preview.myshopify.com) in .env => starts the tunnel + no warning.
• PUBLIC_STORE_DOMAIN is undefined in .env => starts the tunnel + no warning.
• PUBLIC_STORE_DOMAIN is set to mock.shop in .env => skips the tunnel + info banner in terminal about mock.shop.
• PUBLIC_STORE_DOMAIN is empty string in .env => skips the tunnel + info banner in terminal about mock.shop.

[github-actions]

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