Fix null assignment to non-nullable field

Update `BeanRegistrationCodeFragments` package-private constructor
to set `codeFragments` to `this` rather than `null`. The field
is never actually read by `DefaultBeanRegistrationCodeFragments` since
all methods are overridden.

Closes gh-28578

spring-beans/src/main/java/org/springframework/beans/factory/aot/BeanRegistrationCodeFragments.java

@@ -60,10 +60,11 @@ public abstract class BeanRegistrationCodeFragments {
 
 	/**
 	 * Package-private constructor exclusively for
-	 * {@link DefaultBeanRegistrationCodeFragments}.
+	 * {@link DefaultBeanRegistrationCodeFragments}. All methods are overridden
+	 * so {@code this.codeFragments} is never actually used.
 	 */
 	BeanRegistrationCodeFragments() {
-		this.codeFragments = null;
+		this.codeFragments = this;
 	}
 
 	/**