[FIR] Fix substituted synthetic property with overridden getter only

When building the original for a substitution override for a synthetic
property, use the initial setter as fallback when unsubstituting it
returns null. This can happen when a generic class overrides the getter
of a synthetic property of a non-generic class. Then the setter is never
substituted, therefore there is nothing to unsubstitute.

^KT-57168 Fixed

compiler/testData/codegen/box/javaInterop/syntheticPropOverriddenGetter.kt

@@ -0,0 +1,19 @@
+// TARGET_BACKEND: JVM
+// FILE: Component.java
+
+public abstract class Component {
+    public void setPreferredSize(Object preferredSize) {}
+    public Object getPreferredSize() { return new Object(); }
+}
+
+// FILE: ProjectMain.kt
+
+class ComboBox<T>: Component() {
+    override fun getPreferredSize(): Any? = "OK"
+}
+
+fun box(): String {
+    val comboBox = ComboBox<Int>()
+    comboBox.preferredSize = Any()
+    return comboBox.preferredSize as String
+}