Utils return undefined for subscriptions

[LunaticMuch]

Describe the bug
Function getSchemaMap returns undefined for subscriptions instead of an empty object.
Whilst not the biggest deal, it is inconsistent and can lead to parsing issues

To Reproduce
Using a demo schema just load and call getSchemaMap.

• GraphQL schema
  https://gist.github.com/LunaticMuch/8d61f01e0805ac10357082f25bb1e59a

Expected behavior
The expectations is to have

subscriptions: {},
unions: {},
interfaces: {},
enums: {},
inputs: {},

instead of

subscriptions: undefined,
unions: {},
interfaces: {},
enums: {},
inputs: {},

[edno]

@LunaticMuch - I published a fix in @graphql-markdown/utils 1.1.4