Fix #5401 Single-arg method invocations should not fall back to input argument for null return values

Fixes #5401

Author: JoostK

src/main/java/tools/jackson/databind/introspect/AnnotatedMethod.java

@@ -107,8 +107,7 @@ public final Object call(Object[] args) throws Exception {
     @Override
     public final Object call1(Object arg) throws Exception {
         try {
-            Object ret = _invokerUnary.get().invokeExact(arg);
-            return ret == null ? arg : ret;
+            return _invokerUnary.get().invokeExact(arg);
         } catch (final Throwable e) {
             throw sneakyThrow(e);
         }

src/test/java/tools/jackson/databind/deser/creators/JsonCreatorReturningNull4938Test.java

@@ -16,6 +16,26 @@
 public class JsonCreatorReturningNull4938Test
     extends DatabindTestUtil
 {
+    static class NonEmpty {
+      public NonEmptyString nonEmpty;
+    }
+
+    static class NonEmptyString {
+        public final String value;
+
+        @JsonCreator
+        public static NonEmptyString of(String value) {
+            if (value == null || value.isEmpty()) {
+                return null;
+            }
+            return new NonEmptyString(value);
+        }
+
+        private NonEmptyString(String value) {
+            this.value = value;
+        }
+    }
+
     static class Localized3 {
         public final String en;
         public final String de;
@@ -92,6 +112,18 @@ public void addProperty(String key, Object value) {
 
     private final ObjectMapper MAPPER = newJsonMapper();
 
+    @Test
+    void testDeserializeFieldToNullIfCreatorReturnsNull()
+            throws Exception
+    {
+        NonEmpty result = MAPPER.readValue(
+                "{ \"nonEmpty\": \"\" }",
+                NonEmpty.class);
+
+        assertNotNull(result);
+        assertNull(result.nonEmpty);
+    }
+
     @Test
     void testDeserializeToNullWhenAllPropertiesAreNull()
             throws Exception