fix(middleware): avoid sending empty Content-Length

[imlonghao]

Fix caddyserver/cache-handler#104

Tested with

FROM caddy:2.8.4-builder AS builder
RUN xcaddy build --with github.com/darkweak/souin=github.com/imlonghao/souin@fix-empty-response --with github.com/darkweak/souin/plugins/caddy

FROM caddy:2.8.4
COPY --from=builder /usr/bin/caddy /usr/bin/caddy

[netlify]

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[darkweak]

Thank you @imlonghao ! 🚀