FIR: keep nullability when serializing suspend function types

No clue where to put the test for that since diagnostic tests, even
multi-module ones, never touch the metadata serializer. So the test is
a bytecode text one pretending to be about nullability annotations even
though it also affects what resolution in another module will do.

compiler/fir/providers/src/org/jetbrains/kotlin/fir/types/InferenceUtils.kt

@@ -127,7 +127,7 @@ fun ConeKotlinType.suspendFunctionTypeToFunctionTypeWithContinuation(session: Fi
             arrayOf(lastTypeArgument),
             isNullable = false
         ) + lastTypeArgument).toTypedArray(),
-        isNullable = false,
+        isNullable = fullyExpandedType.isNullable,
         attributes = fullyExpandedType.attributes
     )
 }