Java to Kotlin converter: generate nullable type for private non-initialized fields

j2k/tests/testData/ast/enum/enumWithNameField.kt

@@ -1,4 +1,4 @@
 enum class E {
     I
-    private val name: String = 0
+    private val name: String? = null
 }

j2k/tests/testData/ast/field/classChildExtendsBase.kt

@@ -1,7 +1,7 @@
 class Base {
-    private val myFirst: String = 0
+    private val myFirst: String? = null
 }
 
 class Child : Base() {
-    private val mySecond: String = 0
+    private val mySecond: String? = null
 }

j2k/tests/testData/ast/field/privateField.kt

@@ -1,3 +1,3 @@
 class C {
-    private val f: Foo = 0
-}
+    private val f: Foo? = null
+}