fix: lookup returns undefined, not void

[willstott101]

In this context, the lookup function returning undefined is information - it is a union of Some(data) | None.

Void should be used for functions that really return no data.

This has practical implications as in typescript, a property typed string | undefined cannot have string | void assigned to it.

src/entity/create.ts:160:11 - error TS2322: Type 'string | void' is not assignable to type 'string | undefined'.
  Type 'void' is not assignable to type 'string | undefined'.

160           type: mrmime.lookup(name),
              ~~~~


Found 1 error in src/entity/create.ts:160

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