Fixed NullPointerException with IBM SDK 8

The steps to replicate the issue are pretty simple:

1. create a new Maven project (e.g. `ibm-jdk-bug`);

2. add as a dependency `yacl4j-core`, version `0.9.1`;

3. in the same project, create a main class (e.g. `Main.java`) as follows:

```java

import com.yacl4j.core.ConfigurationBuilder;

import yacl4j.repackaged.com.fasterxml.jackson.databind.JsonNode;

public class Main {

	public static void main(String[] args) {

		JsonNode jsonNode = ConfigurationBuilder.newBuilder()
			.source().fromFileOnClasspath("config.yaml")
			.build(JsonNode.class);

		System.out.println(jsonNode);
	}

}

```

4. from command line, run the following Docker command:

```docker
docker run -it --rm --name ibm-jdk-bug -v "$PWD":/ibm-jdk-bug ibmcom/ibmjava:8-sdk /bin/bash -c 'cd ibm-jdk-bug && ./mvnw exec:java -Dexec.mainClass="Main"'
```

5. wait for the unbelievable exception to blow on your face:

```java
java.lang.NullPointerException
	at com.yacl4j.core.util.ConfigurationUtils.emptyConfiguration(ConfigurationUtils.java:33)
	at com.yacl4j.core.ConfigurationBuilder.mergeConfigurationSources(ConfigurationBuilder.java:73)
	at com.yacl4j.core.ConfigurationBuilder.build(ConfigurationBuilder.java:64)
	at Main.main(Main.java:11)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:90)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:55)
	at java.lang.reflect.Method.invoke(Method.java:508)
	at org.codehaus.mojo.exec.ExecJavaMojo$1.run(ExecJavaMojo.java:282)
	at java.lang.Thread.run(Thread.java:785)
```

This is the incriminated yacl4j code:

```java
public class ConfigurationUtils {

	private ConfigurationUtils() { }

	private static final ObjectMapper DEFAULT_OBJECT_MAPPER = Yaml.YAML_OBJECT_MAPPER;

	private static ObjectMapper objectMapper(JsonFactory jsonFactory) {
		return new ObjectMapper(jsonFactory)
				.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
	}

	public static JsonNode emptyConfiguration() {
		return DEFAULT_OBJECT_MAPPER.createObjectNode();
	}

	// ...

	public static class Yaml {

		private Yaml() { }

		private static final ObjectMapper YAML_OBJECT_MAPPER = objectMapper(new YAMLFactory());

		// ...

	}

	// ...

}
```

For some reason I can't really explain, with the IBM SDK 8, the static variable
`DEFAULT_OBJECT_MAPPER` is null! The same works perfectly with Oracle JDK and OpenJDK.

Author: fabriziocucci

yacl4j-core/src/main/java/com/yacl4j/core/util/ConfigurationUtils.java

@@ -20,30 +20,32 @@ public class ConfigurationUtils {
 
 	private ConfigurationUtils() { }
 
-	private static final ObjectMapper DEFAULT_OBJECT_MAPPER = Yaml.YAML_OBJECT_MAPPER;
+	private static ObjectMapper defaultObjectMapper() {
+		return Yaml.YAML_OBJECT_MAPPER;
+	}
 
-	private static ObjectMapper objectMapper(JsonFactory jsonFactory) {
+	private static ObjectMapper buildObjectMapper(JsonFactory jsonFactory) {
 		return new ObjectMapper(jsonFactory)
 				.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
 				.registerModule(new MrBeanModule())
 				.registerModule(new Jdk8Module());
 	}
 
 	public static JsonNode emptyConfiguration() {
-		return DEFAULT_OBJECT_MAPPER.createObjectNode();
+		return defaultObjectMapper().createObjectNode();
 	}
 
 	public static JsonNode fromString(String configuration) {
 		try {
-			return DEFAULT_OBJECT_MAPPER.readValue(configuration, JsonNode.class);
+			return defaultObjectMapper().readValue(configuration, JsonNode.class);
 		} catch (Exception exception) {
 			return TextNode.valueOf(configuration);
 		}
 	}
 
 	public static <T> T toValue(JsonNode configuration, Class<T> configurationClass) {
 		try {
-			return DEFAULT_OBJECT_MAPPER.treeToValue(configuration, configurationClass);
+			return defaultObjectMapper().treeToValue(configuration, configurationClass);
 		} catch (Exception exception) {
 			throw new IllegalStateException(exception);
 		}
@@ -70,7 +72,7 @@ public static class Yaml {
 
 		private Yaml() { }
 
-		private static final ObjectMapper YAML_OBJECT_MAPPER = objectMapper(new YAMLFactory());
+		private static final ObjectMapper YAML_OBJECT_MAPPER = buildObjectMapper(new YAMLFactory());
 
 		public static JsonNode fromInputStream(InputStream configuration) {
 			try {
@@ -98,7 +100,7 @@ public static class Json {
 
 		private Json() { }
 
-		private static final ObjectMapper JSON_OBJECT_MAPPER = objectMapper(null);
+		private static final ObjectMapper JSON_OBJECT_MAPPER = buildObjectMapper(null);
 
 		public static JsonNode fromInputStream(InputStream configuration) {
 			try {