Use initial system for completion if common one is effectively empty

Otherwise we can get unsubstituted type variables as expected types and
 then write wrong information for assertions

 #KT-41470 Fixed

compiler/testData/codegen/box/inference/mapCollectChainWithNullResult.kt

@@ -0,0 +1,38 @@
+// WITH_RUNTIME
+
+@file:OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+
+interface Flow<out T> {
+    fun collect(collector: FlowCollector<T>)
+}
+
+interface FlowCollector<in T> {
+    fun emit(value: T)
+}
+
+fun <T> flow(block: FlowCollector<T>.() -> Unit): Flow<T> =
+    object : Flow<T> {
+        override fun collect(collector: FlowCollector<T>) = collector.block()
+    }
+
+fun <T> Flow<T>.collect(action: (value: T) -> Unit): Unit =
+    collect(object : FlowCollector<T> {
+        override fun emit(value: T) = action(value)
+    })
+
+fun <T, R> Flow<T>.transform(@BuilderInference transform: FlowCollector<R>.(T) -> Unit): Flow<R> =
+    flow { collect { transform(it) } }
+
+fun <T, R> Flow<T>.map(transform: (T) -> R): Flow<R> =
+    transform { emit(transform(it)) }
+
+var result: Any? = "not null"
+
+fun main() {
+    flow<Int> { emit(1) }.map { null }.collect { result = it }
+}
+
+fun box(): String {
+    main()
+    return if (result == null) "OK" else "fail: $result"
+}