Use empty string for null logging prefix

When a logging prefix is null (e.g., when we do not yet know the node
name), this commit causes such loggers to use an empty string marker
instead of an empty prefix (i.e., "[] ") marker.

Author: jasontedor

core/src/main/java/org/elasticsearch/common/logging/PrefixLogger.java

@@ -44,7 +44,7 @@ public String prefix() {
         super(logger, name, null);
         this.prefix = prefix;
 
-        final String actualPrefix = (prefix == null ? "[] " : prefix).intern();
+        final String actualPrefix = (prefix == null ? "" : prefix).intern();
         final Marker actualMarker;
         synchronized (markers) {
             final WeakReference<Marker> marker = markers.get(actualPrefix);

qa/evil-tests/src/test/java/org/elasticsearch/common/logging/EvilLoggerTests.java

@@ -114,7 +114,7 @@ public void testFindAppender() throws IOException {
     public void testPrefixLogger() throws IOException, IllegalAccessException {
         setupLogging("prefix");
 
-        final String prefix = randomAsciiOfLength(16);
+        final String prefix = randomBoolean() ? null : randomAsciiOfLength(16);
         final Logger logger = Loggers.getLogger("prefix", prefix);
         logger.info("test");
         logger.info("{}", "test");
@@ -131,7 +131,11 @@ public void testPrefixLogger() throws IOException, IllegalAccessException {
         final int expectedLogLines = 3;
         assertThat(events.size(), equalTo(expectedLogLines + stackTraceLength));
         for (int i = 0; i < expectedLogLines; i++) {
-            assertThat(events.get(i), startsWith("[" + prefix + "]"));
+            if (prefix == null) {
+                assertThat(events.get(i), startsWith("test"));
+            } else {
+                assertThat(events.get(i), startsWith("[" + prefix + "] test"));
+            }
         }
     }