fixed bug

NULL_FOR_NONNULL_TYPE can represent type mismatch error as well
(later the whole method 'makeTraceInterceptingTypeMismatch' should be removed)

compiler/testData/diagnostics/tests/functionLiterals/returnNull.kt

@@ -0,0 +1,10 @@
+package m
+
+trait Element
+
+fun test(handlers: Map<String, Element.()->Unit>) {
+
+    handlers.getOrElse("name", { null })
+}
+
+fun <K,V> Map<K,V>.getOrElse(key: K, defaultValue: ()-> V) : V = throw Exception("$key $defaultValue")