fix: support nullable enums

[markandrus]

This fixes an error when handling nullable enums:

testEnum:
  type: string
  enum:
    - foo
    - bar
    - null
  nullable: true

  TypeError [Error]: Cannot use 'in' operator to search for 'type' in null
      at file:///openapi-box/src/writer.js:294:22
      at Array.forEach (<anonymous>)
      at writeCompound (file:///openapi-box/src/writer.js:290:10)
      at writeType (file:///openapi-box/src/writer.js:176:7)
      at file:///openapi-box/src/writer.js:354:13
      at Array.forEach (<anonymous>)
      at file:///openapi-box/src/writer.js:351:24
      at CodeBlockWriter._indentBlockInternal (file:///openapi-box/node_modules/.pnpm/code-block-writer@13.0.3/node_modules/code-block-writer/esm/mod.js:261:13)
      at CodeBlockWriter.inlineBlock (file:///openapi-box/node_modules/.pnpm/code-block-writer@13.0.3/node_modules/code-block-writer/esm/mod.js:236:14)
      at writeObject (file:///openapi-box/src/writer.js:350:11)

[dpaez]

LGTM