KT-7695 Java to Kotlin converter incorrectly generates a cast to non-nullable type

#KT-7695 Fixed

j2k/testData/fileOrElement/typeCastExpression/castNullable.java

@@ -0,0 +1,12 @@
+class A {
+    private String foo(Object o, boolean b) {
+        if (b) return (String) o;
+        return "";
+    }
+
+    void bar() {
+        if (foo(null, true) == null) {
+            System.out.println("null");
+        }
+    }
+}

j2k/testData/fileOrElement/typeCastExpression/castNullable.kt

@@ -0,0 +1,12 @@
+class A {
+    private fun foo(o: Any?, b: Boolean): String? {
+        if (b) return o as String?
+        return ""
+    }
+
+    fun bar() {
+        if (foo(null, true) == null) {
+            println("null")
+        }
+    }
+}