Expose Elasticsearch API nullability information to Kotlin compiler. (#43912) (#44518)

This change allows the Kotlin compiler to type check methods annotated with the
org.elasticsearch.common.Nullable annotation in Elasticsearch Java
APIs as described in: https://kotlinlang.org/docs/reference/java-interop.html#jsr-305-support.

(cherry picked from commit 0d0485a)

Author: mark-vieira

libs/core/build.gradle

@@ -73,6 +73,9 @@ publishing {
 }
 
 dependencies {
+    // This dependency is used only by :libs:core for null-checking interop with other tools
+    compileOnly "com.google.code.findbugs:jsr305:3.0.2"
+
     testCompile "com.carrotsearch.randomizedtesting:randomizedtesting-runner:${versions.randomizedrunner}"
     testCompile "junit:junit:${versions.junit}"
     testCompile "org.hamcrest:hamcrest:${versions.hamcrest}"

libs/core/src/main/java/org/elasticsearch/common/Nullable.java

@@ -19,6 +19,8 @@
 
 package org.elasticsearch.common;
 
+import javax.annotation.CheckForNull;
+import javax.annotation.meta.TypeQualifierNickname;
 import java.lang.annotation.Documented;
 import java.lang.annotation.ElementType;
 import java.lang.annotation.Retention;
@@ -33,6 +35,8 @@
  *
  */
 @Documented
+@TypeQualifierNickname
+@CheckForNull
 @Retention(RetentionPolicy.RUNTIME)
 @Target({ElementType.PARAMETER, ElementType.FIELD, ElementType.METHOD})
 public @interface Nullable {