Make it possible to unset values in update-repo

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The underlying setters interpret NULL as 'unset this key', but
we never pass NULL. Since empty strings are not useful values
for title or default-branch or redirect-url, just interpret
an explicit empty value as 'unset this key'. E.g. to unset
the redirect url, use

flatpak build-update-repo --redirect-url= ~/my-repo/

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