Split up TapReporter function into two functions

Suggested by @tombentley.

source/ceylon/test/engine/internal/Runner.ceylon

@@ -56,24 +56,15 @@ shared class Runner() {
         if (exists socket) {
             testListeners.add(TestEventPublisher(socket.write));
         } else if (exists tap = options.tap) {
-            void write(String? line);
             if (tap == "-") {
-                write = void(String? line) {
-                    if (exists line) {
-                        print(line);
-                    }
-                };
+                testListeners.add(TapReporter());
             } else {
                 value writer = FileWriter(tap);
-                write = void(String? line) {
-                    if (exists line) {
-                        writer.writeLine(line);
-                    } else {
-                        writer.destroy(null);
-                    }
-                };
+                testListeners.add(TapReporter {
+                        void write(String line) => writer.writeLine(line);
+                        void close() => writer.destroy(null);
+                    });
             }
-            testListeners.add(TapReporter(write));
         } else {
             testListeners.add(TestLoggingListener(options.colorReset, options.colorGreen, options.colorRed));
         }

source/ceylon/test/reporter/TapReporter.ceylon

@@ -108,12 +108,12 @@ import ceylon.test.engine {
          at com.redhat.ceylon.launcher.Launcher.main(Launcher.java:21)
    ...
  ~~~"
-shared class TapReporter(write) satisfies TestListener {
+shared class TapReporter(write = print, close = noop) satisfies TestListener {
 
-    "A function that logs the given line, for example by writing to standard output.
-     The function is called with a [[null]] argument at the end to signal the end of reporting;
-     this may be used to close an underlying stream if necessary."
-    void write(String? line);
+    "A function that logs the given line, for example [[print]]."
+    void write(String line);
+    "A function that is called at the end of reporting and may, for example, close an underlying stream."
+    void close();
 
     variable Integer count = 1;
 
@@ -123,7 +123,7 @@ shared class TapReporter(write) satisfies TestListener {
 
     shared actual void testRunFinished(TestRunFinishedEvent event) {
         write("1..`` count - 1 ``");
-        write(null); // close stream
+        close();
     }
 
     shared actual void testFinished(TestFinishedEvent event) => writeProtocol(event);