ValueExpressionResolver should be annotated with @Nullable

[asinbow]

Describe the bug
As @NonNullApi used in the package, ValueExpressionResolver is annotated with both input and output @nonnull. But this is not true at runtime, which makes it not friendly to Kotlin.

Environment
v1.11.4

To Reproduce

@Component
class MyComponent {
  @Timed
  void myMethod(@MeterTag("slug", expression = "#this" String slug) { }
}

@Autowired private myComponent: MyComponent;

myComponent.myMethod(null);
// Then a null parameter will be sent to the implementation ValueExpressionResolver.

java.lang.NullPointerException: Parameter specified as non-null is null: method SpelValueExpressionResolver.resolve, parameter parameter
	at com.flexport.bookings.config.MetricsConfig$SpelValueExpressionResolver.resolve(MetricsConfig.kt)
	at io.micrometer.core.aop.MeterTagAnnotationHandler.resolveTagValue(MeterTagAnnotationHandler.java:70)
	at io.micrometer.core.aop.MeterTagAnnotationHandler.lambda$new$1(MeterTagAnnotationHandler.java:52)
	at io.micrometer.common.annotation.AnnotationHandler.addAnnotatedArguments(AnnotationHandler.java:141)
	at io.micrometer.common.annotation.AnnotationHandler.addAnnotatedParameters(AnnotationHandler.java:96)
	at io.micrometer.core.aop.TimedAspect.recordBuilder(TimedAspect.java:258)
	at io.micrometer.core.aop.TimedAspect.record(TimedAspect.java:241)
	at io.micrometer.core.aop.TimedAspect.processWithTimer(TimedAspect.java:234)
	at io.micrometer.core.aop.TimedAspect.perform(TimedAspect.java:202)
	at io.micrometer.core.aop.TimedAspect.timedMethod(TimedAspect.java:194)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethodWithGivenArgs(AbstractAspectJAdvice.java:637)
	at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethod(AbstractAspectJAdvice.java:627)
	at org.springframework.aop.aspectj.AspectJAroundAdvice.invoke(AspectJAroundAdvice.java:71)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184)
	at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:750)
	at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:97)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184)
	at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:750)
	at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:702)

Expected behavior
ValueExpressionResolver should defined like this:

public interface ValueExpressionResolver {
  @Nullable String resolve(String expression, @Nullable Object parameter);
}

Another option is to ensure micrometer doesn't pass in null to the parameter, because it doesn't make much sense..