Make rootObject parameter consistently nullable

MethodBasedEvaluationContext and CacheEvaluationContext should allow a
nullable rootObject constructor parameter like
StandardEvaluationContext does.

Closes gh-35206

Author: sdeleuze

spring-context/src/main/java/org/springframework/cache/interceptor/CacheEvaluationContext.java

@@ -48,7 +48,7 @@ class CacheEvaluationContext extends MethodBasedEvaluationContext {
 	private final Set<String> unavailableVariables = new HashSet<>(1);
 
 
-	CacheEvaluationContext(Object rootObject, Method method, @Nullable Object[] arguments,
+	CacheEvaluationContext(@Nullable Object rootObject, Method method, @Nullable Object[] arguments,
 			ParameterNameDiscoverer parameterNameDiscoverer) {
 
 		super(rootObject, method, arguments, parameterNameDiscoverer);

spring-context/src/main/java/org/springframework/context/expression/MethodBasedEvaluationContext.java

@@ -53,7 +53,7 @@ public class MethodBasedEvaluationContext extends StandardEvaluationContext {
 	private boolean argumentsLoaded = false;
 
 
-	public MethodBasedEvaluationContext(Object rootObject, Method method, @Nullable Object[] arguments,
+	public MethodBasedEvaluationContext(@Nullable Object rootObject, Method method, @Nullable Object[] arguments,
 			ParameterNameDiscoverer parameterNameDiscoverer) {
 
 		super(rootObject);