Partial fix for KT-3698: properly write field initializer

compiler/testData/codegen/box/namespace/nullablePrimitiveNoFieldInitializer.kt

@@ -0,0 +1,11 @@
+val zint : Int? = 1
+val zlong : Long? = 2
+val zbyte : Byte? = 3
+val zshort : Short? = 4
+val zchar : Char? = 'c'
+val zdouble : Double? = 1.0
+val zfloat : Float? = 2.0
+
+fun box(): String {
+  return "OK"
+}