Introduce null-safety of Spring Framework API

This commit introduces 2 new @Nullable and @NonNullApi
annotations that leverage JSR 305 (dormant but available via
Findbugs jsr305 dependency and already used by libraries
like OkHttp) meta-annotations to specify explicitly
null-safety of Spring Framework parameters and return values.

In order to avoid adding too much annotations, the
default is set at package level with @NonNullApi and
@Nullable annotations are added when needed at parameter or
return value level. These annotations are intended to be used
on Spring Framework itself but also by other Spring projects.

@Nullable annotations have been introduced based on Javadoc
and search of patterns like "return null;". It is expected that
nullability of Spring Framework API will be polished with
complementary commits.

In practice, this will make the whole Spring Framework API
null-safe for Kotlin projects (when KT-10942 will be fixed)
since Kotlin will be able to leverage these annotations to
know if a parameter or a return value is nullable or not. But
this is also useful for Java developers as well since IntelliJ
IDEA, for example, also understands these annotations to
generate warnings when unsafe nullable usages are detected.

Issue: SPR-15540

spring-aop/src/main/java/org/aopalliance/aop/AspectException.java

@@ -16,6 +16,8 @@
 
 package org.aopalliance.aop;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Superclass for all AOP infrastructure exceptions.
  * Unchecked, as such exceptions are fatal and end user
@@ -41,7 +43,7 @@ public class AspectException extends RuntimeException {
 	 * @param message the exception message
 	 * @param cause the root cause, if any
 	 */
-	public AspectException(String message, Throwable cause) {
+	public AspectException(String message, @Nullable Throwable cause) {
 		super(message, cause);
 	}
 

spring-aop/src/main/java/org/aopalliance/intercept/MethodInterceptor.java

@@ -16,6 +16,8 @@
 
 package org.aopalliance.intercept;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Intercepts calls on an interface on its way to the target. These
  * are nested "on top" of the target.
@@ -52,6 +54,7 @@ public interface MethodInterceptor extends Interceptor {
 	 * @throws Throwable if the interceptors or the target object
 	 * throws an exception
 	 */
+	@Nullable
 	Object invoke(MethodInvocation invocation) throws Throwable;
 
 }