RedundantLetInspection: fix false positive with nullable receiver extension call

#KT-31601 Fixed

idea/testData/inspectionsLocal/simpleRedundantLet/extensionCall.kt

@@ -0,0 +1,6 @@
+// WITH_RUNTIME
+fun String.foo() {}
+
+fun test(s: String?) {
+    s?.let<caret> { it.foo() }
+}

idea/testData/inspectionsLocal/simpleRedundantLet/extensionCall.kt.after

@@ -0,0 +1,6 @@
+// WITH_RUNTIME
+fun String.foo() {}
+
+fun test(s: String?) {
+    s?.foo()
+}

idea/testData/inspectionsLocal/simpleRedundantLet/extensionWithNullableReceiverCall.kt

@@ -0,0 +1,10 @@
+// WITH_RUNTIME
+// PROBLEM: none
+class Optional<out T>(val value: T)
+
+fun Any?.foo() = println("foo: $this")
+
+fun main() {
+    val b: Optional<Any?>? = Optional(null)
+    b?.let<caret> { it.value.foo() }
+}

idea/testData/inspectionsLocal/simpleRedundantLet/extensionWithNullableReceiverCall2.kt

@@ -0,0 +1,10 @@
+// WITH_RUNTIME
+// PROBLEM: none
+class Optional<out T>(val value: T)
+
+val Any?.foo get() = println("foo: $this")
+
+fun main() {
+    val b: Optional<Any?>? = Optional(null)
+    b?.let<caret> { it.value.foo }
+}

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 }
+}