Avoid creating map allocator when writing empty maps to StreamOutput (e…

…lastic#105071)

It's in the title. We already have the size here and allocating the
iterator isn't free. In fact it's 10G of allocations during http_logs
indexing that we can avoid with a simple condition.

server/src/main/java/org/elasticsearch/common/io/stream/StreamOutput.java

@@ -596,10 +596,13 @@ public final <K extends Writeable, V extends Writeable> void writeMap(final Map<
      * @param valueWriter The value writer
      */
     public final <K, V> void writeMap(final Map<K, V> map, final Writer<K> keyWriter, final Writer<V> valueWriter) throws IOException {
-        writeVInt(map.size());
-        for (final Map.Entry<K, V> entry : map.entrySet()) {
-            keyWriter.write(this, entry.getKey());
-            valueWriter.write(this, entry.getValue());
+        int size = map.size();
+        writeVInt(size);
+        if (size > 0) {
+            for (final Map.Entry<K, V> entry : map.entrySet()) {
+                keyWriter.write(this, entry.getKey());
+                valueWriter.write(this, entry.getValue());
+            }
         }
     }