RedundantLetInspection: fix false positive with nullable receiver extension call

#KT-31601 Fixed

idea/testData/inspectionsLocal/simpleRedundantLet/extensionWithNullableReceiverCall3.kt

@@ -0,0 +1,14 @@
+// WITH_RUNTIME
+// PROBLEM: none
+class A(val b: B?)
+class B
+class C(val d: D)
+class D
+
+fun B?.getC(): C {
+    return C(D())
+}
+
+fun test(a: A?) {
+    a?.let<caret> { it.b.getC().d }
+}