Migrate from @CheckForNull to Checker Framework @Nullable.

A future step will migrate from these and our already existing usages of the Checker Framework `@Nullable` to the JSpecify `@Nullable`.

This is the next step toward [using JSpecify in Guava](jspecify/jspecify#239 (comment)).

RELNOTES=Migrated from `@CheckForNull` to Checker Framework `@Nullable`. Most tools recognize both annotations, so we expect this to be a no-op in almost all cases.
PiperOrigin-RevId: 708617636

Author: cpovirk

Diff for: android/guava-testlib/src/com/google/common/testing/ClassSanityTester.java

@@ -50,7 +50,6 @@
 import java.util.List;
 import java.util.Map.Entry;
 import java.util.Set;
-import javax.annotation.CheckForNull;
 import junit.framework.Assert;
 import junit.framework.AssertionFailedError;
 import org.jspecify.annotations.NullUnmarked;
@@ -657,8 +656,7 @@ private FreshValueGenerator newFreshValueGenerator() {
     FreshValueGenerator generator =
         new FreshValueGenerator() {
           @Override
-          @CheckForNull
-          Object interfaceMethodCalled(Class<?> interfaceType, Method method) {
+          @Nullable Object interfaceMethodCalled(Class<?> interfaceType, Method method) {
             return getDummyValue(TypeToken.of(interfaceType).method(method).getReturnType());
           }
         };
@@ -737,8 +735,7 @@ private List<Object> getDummyArguments(Invokable<?, ?> invokable)
     return args;
   }
 
-  @CheckForNull
-  private <T> T getDummyValue(TypeToken<T> type) {
+  private <T> @Nullable T getDummyValue(TypeToken<T> type) {
     Class<? super T> rawType = type.getRawType();
     @SuppressWarnings("unchecked") // Assume all default values are generics safe.
     T defaultValue = (T) defaultValues.getInstance(rawType);

Diff for: android/guava-testlib/src/com/google/common/testing/FreshValueGenerator.java

@@ -120,7 +120,6 @@
 import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.regex.Pattern;
-import javax.annotation.CheckForNull;
 import org.jspecify.annotations.NullUnmarked;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
@@ -294,8 +293,8 @@ private final class FreshInvocationHandler extends AbstractInvocationHandler {
     }
 
     @Override
-    @CheckForNull
-    protected Object handleInvocation(Object proxy, Method method, @Nullable Object[] args) {
+    protected @Nullable Object handleInvocation(
+        Object proxy, Method method, @Nullable Object[] args) {
       return interfaceMethodCalled(interfaceType, method);
     }
 
@@ -320,8 +319,7 @@ public String toString() {
   }
 
   /** Subclasses can override to provide different return value for proxied interface methods. */
-  @CheckForNull
-  Object interfaceMethodCalled(Class<?> interfaceType, Method method) {
+  @Nullable Object interfaceMethodCalled(Class<?> interfaceType, Method method) {
     throw new UnsupportedOperationException();
   }