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e0d4df21 - [ppr] Narrow condition for fallback shell generation at runtime (#79565)

[ppr] Narrow condition for fallback shell generation at runtime (#79565) This addresses a bug where a fallback shell was generated at runtime when it was not expected to do so. This was an issue with how the `x-now-route-matches` header was parsed and was incorrectly assuming that a falsy header was the same as comparing it to a string. This correctly only generates the fallback shell if the header is present and was an empty string as was sent by Vercel. Co-authored-by: Hendrik Liebau <mail@hendrik-liebau.de>