[FIR] KT-56723: Ensure safe access over non-expressions is always Unit

^KT-56723 Fixed

compiler/testData/codegen/box/unit/kt56723.kt

@@ -0,0 +1,27 @@
+fun foo(f: () -> Unit, returnIfOk: String): String {
+    val string = f().toString()
+
+    return if (string == "kotlin.Unit") {
+        returnIfOk
+    } else {
+        "FAIL: $string;"
+    }
+}
+
+class Wrapper(var s: String)
+
+fun box(): String {
+    val w: Wrapper? = Wrapper("Test")
+
+    val lambda = {
+        w?.s = "X"
+    }
+
+    val w2: Wrapper? = null
+
+    val lambda2 = {
+        w2?.s = "X"
+    }
+
+    return foo(lambda, "O") + foo(lambda2, "K")
+}