Fix the errors reported by NullAway (line#2776)

Motivation:

Before adopting NullAway(line#1680) or other static analysis tools.
Manually run it and clean up errors.

Modifications:

- Fix all true positives reported by NullAway

Result:

Less NullAway errors

build.gradle

@@ -2,7 +2,7 @@ import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
 plugins {
     id 'com.google.osdetector' version '1.6.2' apply false
-    // If you want to change `org.jetbrains.kotlin.jvm` version, 
+    // If you want to change `org.jetbrains.kotlin.jvm` version,
     // you also need to change `org.jetbrains.kotlin:kotlin-allopen` version in `dependencies.yml`.
     id 'org.jetbrains.kotlin.jvm' version '1.3.72' apply false
 

core/src/main/java/com/linecorp/armeria/client/RefreshingAddressResolver.java

@@ -327,7 +327,7 @@ public void run() {
                 servedFromCache = false;
                 numAttemptsSoFar = 1;
 
-                if (entry.address().equals(address) && entry.ttlMillis() == ttlMillis) {
+                if (address.equals(entry.address()) && entry.ttlMillis() == ttlMillis) {
                     scheduleRefresh(ttlMillis);
                 } else {
                     // Replace the old entry with the new one.

core/src/main/java/com/linecorp/armeria/common/MediaType.java

@@ -1214,6 +1214,7 @@ private static final class KnownTypesByString {
                       .distinct()
                       .collect(toImmutableMap(MediaType::toString, Function.identity(), (a, b) -> a));
 
+        @Nullable
         static MediaType get(String input) {
             return KNOWN_TYPES_BY_STRING.get(input);
         }

core/src/main/java/com/linecorp/armeria/internal/common/DefaultTimeoutController.java

@@ -269,6 +269,7 @@ ScheduledFuture<?> timeoutFuture() {
         return timeoutFuture;
     }
 
+    @Nullable
     @VisibleForTesting
     TimeoutTask timeoutTask() {
         return timeoutTask;

core/src/main/java/com/linecorp/armeria/internal/common/util/ObjectCollectingUtil.java

@@ -42,6 +42,7 @@ public final class ObjectCollectingUtil {
      * The {@link Class} instance of {@code reactor.core.publisher.Mono} of
      * <a href="https://projectreactor.io/">Project Reactor</a>.
      */
+    @Nullable
     private static final Class<?> MONO_CLASS;
 
     static {

core/src/main/java/com/linecorp/armeria/internal/server/annotation/AnnotationUtil.java

@@ -220,14 +220,14 @@ static <T extends Annotation> List<T> find(AnnotatedElement element, Class<T> an
 
     private static <T extends Annotation> void findMetaAnnotations(
             Builder<T> builder, Annotation annotation,
-            Class<T> annotationType, Class<? extends Annotation> containerType) {
+            Class<T> annotationType, @Nullable Class<? extends Annotation> containerType) {
         findMetaAnnotations(builder, annotation, annotationType, containerType,
                             Collections.newSetFromMap(new IdentityHashMap<>()));
     }
 
     private static <T extends Annotation> boolean findMetaAnnotations(
             Builder<T> builder, Annotation annotation,
-            Class<T> annotationType, Class<? extends Annotation> containerType,
+            Class<T> annotationType, @Nullable Class<? extends Annotation> containerType,
             Set<Class<? extends Annotation>> visitedAnnotationTypes) {
 
         final Class<? extends Annotation> actualAnnotationType = annotation.annotationType();

core/src/main/java/com/linecorp/armeria/server/docs/DocService.java

@@ -301,6 +301,7 @@ private static FieldInfo addFieldDocString(NamedTypeInfo parent, FieldInfo field
                              docString(parent.name() + '/' + field.name(), field.docString(), docStrings));
     }
 
+    @Nullable
     private static String docString(
             String key, @Nullable String currentDocString, Map<String, String> docStrings) {
         return currentDocString != null ? currentDocString : docStrings.get(key);