GH-10083: Apply Nullability to groovy module

Related to: #10083

* Apply Nullability to `groovy.dsl` package

Signed-off-by: Tran Ngoc Nhan <ngocnhan.tran1996@gmail.com>

Author: ngocnhan-tran1996

spring-integration-groovy/src/main/groovy/org/springframework/integration/groovy/dsl/package-info.groovy

@@ -1,4 +1,5 @@
 /**
  * Provides Spring Integration Groovy DSL.
  */
+@org.jspecify.annotations.NullMarked
 package org.springframework.integration.groovy.dsl

spring-integration-groovy/src/main/java/org/springframework/integration/groovy/GroovyScriptExecutingMessageProcessor.java

@@ -32,6 +32,7 @@
 import groovy.transform.CompileStatic;
 import org.codehaus.groovy.control.CompilerConfiguration;
 import org.codehaus.groovy.control.customizers.ASTTransformationCustomizer;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.BeanUtils;
 import org.springframework.beans.factory.BeanFactory;
@@ -75,8 +76,9 @@ public class GroovyScriptExecutingMessageProcessor extends AbstractScriptExecuti
 
 	private boolean compileStatic;
 
-	private CompilerConfiguration compilerConfiguration;
+	private @Nullable CompilerConfiguration compilerConfiguration;
 
+	@SuppressWarnings("NullAway.Init")
 	private volatile Class<?> scriptClass;
 
 	/**

spring-integration-groovy/src/main/java/org/springframework/integration/groovy/VariableBindingGroovyObjectCustomizerDecorator.java

@@ -21,6 +21,7 @@
 import groovy.lang.Binding;
 import groovy.lang.GroovyObject;
 import groovy.lang.Script;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.scripting.groovy.GroovyObjectCustomizer;
 import org.springframework.util.Assert;
@@ -33,9 +34,9 @@
  */
 class VariableBindingGroovyObjectCustomizerDecorator implements GroovyObjectCustomizer {
 
-	private volatile Map<String, ?> variables;
+	private volatile @Nullable Map<String, ?> variables;
 
-	private volatile GroovyObjectCustomizer customizer;
+	private volatile @Nullable GroovyObjectCustomizer customizer;
 
 	public void setVariables(Map<String, ?> variables) {
 		this.variables = variables;

spring-integration-groovy/src/main/java/org/springframework/integration/groovy/config/package-info.java

@@ -1,4 +1,5 @@
 /**
  * Provides classes for configuration - parsers, namespace handlers, factory beans.
  */
+@org.jspecify.annotations.NullMarked
 package org.springframework.integration.groovy.config;

spring-integration-groovy/src/main/java/org/springframework/integration/groovy/package-info.java

@@ -1,4 +1,5 @@
 /**
  * Base package for Groovy support.
  */
+@org.jspecify.annotations.NullMarked
 package org.springframework.integration.groovy;