Improve JavaDoc and Test for config ACCEPT_EMPTY_STRING_AS_NULL_OBJECT wrt special cases (#4012)

Author: JooHyukKim

src/main/java/com/fasterxml/jackson/databind/DeserializationFeature.java

@@ -358,6 +358,9 @@ public enum DeserializationFeature implements ConfigFeature
      * whether they can be coerced depends on
      * {@link MapperFeature#ALLOW_COERCION_OF_SCALARS}.
      *<p>
+     * IMPORTANT: This feature might work even when an empty string {@code ""}
+     * may be a valid value for some types.
+     *<p>
      * Feature is disabled by default.
      */
     ACCEPT_EMPTY_STRING_AS_NULL_OBJECT(false),

src/test/java/com/fasterxml/jackson/databind/deser/jdk/LocaleDeser4009Test.java

@@ -0,0 +1,47 @@
+package com.fasterxml.jackson.databind.deser.jdk;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.BaseMapTest;
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.ObjectReader;
+import java.util.Locale;
+
+// [databind#4009] Locale "" is deserialised as NULL if ACCEPT_EMPTY_STRING_AS_NULL_OBJECT is true
+public class LocaleDeser4009Test extends BaseMapTest 
+{
+
+    static class MyPojo {
+        public String field;
+    }
+    final ObjectMapper mapper = newJsonMapper();
+    
+    final ObjectReader DISABLED_READER = mapper.reader()
+            .without(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);
+    
+    final ObjectReader ENABLED_READER = mapper.reader()
+            .with(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);
+
+    public void testPOJOWithFeatureDisabled() 
+    {
+        assertThrows(JsonProcessingException.class, () -> {
+            DISABLED_READER.readValue("\"\"", MyPojo.class);
+        });
+    }
+
+    public void testPOJOWithFeatureEnabled() throws Exception 
+    {
+        assertNull(ENABLED_READER.readValue("\"\"", MyPojo.class));
+    }
+
+    public void testLocaleWithFeatureDisabled() throws Exception 
+    {
+        assertEquals(Locale.ROOT, DISABLED_READER.readValue("\"\"", Locale.class));
+    }
+
+    public void testLocaleWithFeatureEnabled() throws Exception 
+    {
+        assertNull(ENABLED_READER.readValue("\"\"", Locale.class));
+    }
+}