Add test for obsolete issue

#KT-38420 Obsolete

compiler/testData/diagnostics/testsWithStdLib/coroutines/inference/kt38420.kt

@@ -0,0 +1,10 @@
+// !USE_EXPERIMENTAL: kotlin.RequiresOptIn
+
+@OptIn(ExperimentalStdlibApi::class)
+val list: List<String> = buildList {
+    val inner: List<String> = maybe() ?: emptyList()
+
+    addAll(inner)
+}
+
+fun maybe(): List<String>? = null

compiler/testData/diagnostics/testsWithStdLib/coroutines/inference/kt38420.txt

@@ -0,0 +1,4 @@
+package
+
+@kotlin.OptIn(markerClass = {kotlin.ExperimentalStdlibApi::class}) public val list: kotlin.collections.List<kotlin.String>
+public fun maybe(): kotlin.collections.List<kotlin.String>?