Add a specialized Optional serializer actually serializing values

Description

Introduces a specialized Optional serializer because Jackson was only using public getter-like functions, which happened to be only isPresent and isEmpty, making any results useless.

Changes

None.

Additions

• An Optional serializer
• Registration in the Core class

Test and Review

To be filled in by the reviewers

• All of the methods are commented to expectation

• The methods are tested to satisfaction

• There are no unnecessary files present in the MR

• The continuous integration has no problems with the MR

• The MR is filled in as requested (including labels, milestones, and reviewers)

• The documentation is up-to-date

• All nullable parameters are marked as such

core/src/main/java/nl/tudelft/ewi/auta/core/response/OptionalSerializer.java

+package nl.tudelft.ewi.auta.core.response;
+
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.databind.SerializerProvider;
+import com.fasterxml.jackson.databind.ser.std.StdSerializer;
+import org.springframework.stereotype.Service;
+
+import java.io.IOException;
+import java.util.Optional;
+
+/**
+ * A serializer for {@link Optional}s.
+ *
+ * By default Jackson only touches what it can recognize as getters, which in the case of Optional
+ * appear to be {@link Optional#isEmpty()} and {@link Optional#isPresent()}. This serializer
+ * instead serializes the value if present and outputs {@code null} otherwise.
+ */
+@Service
+public class OptionalSerializer extends StdSerializer<Optional> {
+    /**
+     * Creates a new {@code Optional} serializer.
+     */
+    public OptionalSerializer() {
+        super(Optional.class);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void serialize(
+            final Optional optional, final JsonGenerator gen, final SerializerProvider provider
+    ) throws IOException {
+        if (optional.isPresent()) {
+            final var value = optional.get();
+            provider.findValueSerializer(value.getClass()).serialize(value, gen, provider);
+        } else {
+            gen.writeNull();
+        }
+    }
+}