chore(deps): update dependency com.uber.nullaway:nullaway to v0.13.8

pom.xml

@@ -505,7 +505,7 @@
                 <path>
                   <groupId>com.uber.nullaway</groupId>
                   <artifactId>nullaway</artifactId>
-                  <version>0.13.7</version>
+                  <version>0.13.8</version>
                 </path>
               </annotationProcessorPaths>
             </configuration>

prometheus-metrics-instrumentation-caffeine/src/main/java/io/prometheus/metrics/instrumentation/caffeine/CacheMetricsCollector.java

@@ -20,6 +20,7 @@
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 import java.util.stream.Collectors;
+import javax.annotation.Nullable;
 
 /**
  * Collect metrics from Caffeine's com.github.benmanes.caffeine.cache.Cache.
@@ -149,6 +150,7 @@ public void addCache(String cacheName, AsyncCache<?, ?> cache) {
    *
    * @param cacheName cache to be removed
    */
+  @Nullable
   public Cache<?, ?> removeCache(String cacheName) {
     return children.remove(cacheName);
   }

prometheus-metrics-instrumentation-guava/src/main/java/io/prometheus/metrics/instrumentation/guava/CacheMetricsCollector.java

@@ -16,6 +16,7 @@
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
+import javax.annotation.Nullable;
 
 /**
  * Collect metrics from Guava's com.google.common.cache.Cache.
@@ -100,6 +101,7 @@ public void addCache(String cacheName, Cache<?, ?> cache) {
    *
    * @param cacheName cache to be removed
    */
+  @Nullable
   public Cache<?, ?> removeCache(String cacheName) {
     return children.remove(cacheName);
   }