Generate map defaults for object values on Map-typed scalars

[nboisnea]

• Object literal default values on input fields typed as a scalar mapped to Map (or Object / Any) now generate a map initializer instead of failing with NoSuchElementException:
  • Java: Collections.emptyMap() for {}, otherwise new LinkedHashMap<K, V>(){{put(...);}}
  • Kotlin: emptyMap() / mapOf("key" to value, ...) (mutableMapOf for MutableMap)
• Nested object and list values are generated recursively, using the map's value type
• List defaults now pass the element type to their elements, so lists of maps are supported too ([JSON] = [{}])
• Object literals on any other non-input type fail with an explicit message (X cannot be created from ObjectValue{...}, expected an input type or a Map), similarly to BigDecimal/Currency/Locale checks
• Java: string values are now also generated for Object targets, so {name: "x"} doesn't become put("name", null)

Fixes #982