FIR deserializer: read nullability & add constructors

compiler/fir/resolve/testData/loadCompiledKotlin/class/NestedClassExtendNestedClass.txt

@@ -1,8 +1,14 @@
 public final class Outer : R|kotlin/Any| {
+    public constructor()
+
     public open class Nested1 : R|kotlin/Any| {
+        public constructor()
+
     }
 
     public final class Nested2 : R|test/Outer.Nested1| {
+        public constructor()
+
     }
 
 }