refactor: improve handling of null body in response

[usualoma]

This PR includes a minor bug fix and refactoring related to new Response(null), which was improved in #333.

Summary

• fix: Fix an issue where Content-Type header was unnecessarily set on responses with a null body (e.g., new Response(null))
  • https://github.com/honojs/node-server/pull/341/changes#diff-0a9945c399ef4b66209b330f33399ac079477f017039f62d33e7d8296163469dL160-R161
• fix: Per the Fetch spec, responses with a null body should not include a Content-Type header
  • https://github.com/honojs/node-server/pull/341/changes#diff-0a9945c399ef4b66209b330f33399ac079477f017039f62d33e7d8296163469dL265-R268
  • https://github.com/honojs/node-server/pull/341/changes#diff-8c103f2199265c6a0659504dac715a5bc176ab1f494fc64fb06a5f4a814094c0L66-R68
• refactor: Define defaultContentType as a constant and pass it explicitly to buildOutgoingHttpHeaders
  • https://github.com/honojs/node-server/pull/341/changes#diff-8c103f2199265c6a0659504dac715a5bc176ab1f494fc64fb06a5f4a814094c0R6

[usualoma]

Hi @yusukebe
Would you mind reviewing this?

[yusukebe]

LGTM!

[yusukebe]

@usualoma

Great. Well noticed! Thank you!