Are there any conditions that do not allow a Module to register with ObjectMapper in Jackson?

I have the following super simple code to test the functionality of a custom serializer in Jackson. For some reason, the module does not register to the ObjectMapper (at least I don't see it register based on the results). Are there any conditions that do not allow a Module to register with ObjectMapper?

import java.io.IOException;

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.module.SimpleModule;

public class TestJacksonCustomSerializer {

    public class TheClass {

        public String emptyString = null;
        public String[] emptyStringArray= null;

        public TheClass() {
            
        }
        
        public String getEmptyString() {
            return emptyString;
        }
        
        public String[] getEmptyStringArray() {
            return emptyStringArray;
        }
    }
    
    public class NullStringJsonSerializer extends JsonSerializer<String> {

        @Override
        public void serialize(String string, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
            System.out.println("Processing value: " + String.valueOf(string));
            if (string == null) jsonGenerator.writeString("");          
        }
    }
    
    public static void main(String[] args) throws Exception {
        
        TestJacksonCustomSerializer testJacksonCustomSerializer = new TestJacksonCustomSerializer();

        SimpleModule simpleModule = new SimpleModule();
        simpleModule.addSerializer(String.class, testJacksonCustomSerializer.new NullStringJsonSerializer());
        
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.registerModule(simpleModule);
        
        System.out.println("Number of registered modules: " + String.valueOf(objectMapper.getRegisteredModuleIds().size()));
    
        TheClass theClass = testJacksonCustomSerializer.new TheClass();
        
        System.out.println(objectMapper.writeValueAsString(theClass));
        
    }
}

Output:

Number of registered modules: 0
{"emptyString":null,"emptyStringArray":null}

I am using jackson 2.12.7

Your serializer is not invoked because Jackson determine serializer by type of value, not by type of property. Your value is null so NullSeriaizer is used.