[java] JSpecify annotations for FileDetector (#16482)

Co-authored-by: Diego Molina <diemol@users.noreply.github.com>

Author: mk868

java/src/org/openqa/selenium/remote/FileDetector.java

@@ -18,8 +18,11 @@
 package org.openqa.selenium.remote;
 
 import java.io.File;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 /** Used for identifying whether a sequence of chars represents the path to a file. */
+@NullMarked
 public interface FileDetector {
-  File getLocalFile(CharSequence... keys);
+  @Nullable File getLocalFile(CharSequence... keys);
 }

java/src/org/openqa/selenium/remote/LocalFileDetector.java

@@ -19,14 +19,17 @@
 
 import java.io.File;
 import java.util.logging.Logger;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 /** Detects files on the local disk. */
+@NullMarked
 public class LocalFileDetector implements FileDetector {
 
   private static final Logger LOG = Logger.getLogger(LocalFileDetector.class.getName());
 
   @Override
-  public File getLocalFile(CharSequence... keys) {
+  public @Nullable File getLocalFile(CharSequence... keys) {
     StringBuilder builder = new StringBuilder();
     for (CharSequence chars : keys) {
       builder.append(chars);

java/src/org/openqa/selenium/remote/UselessFileDetector.java

@@ -18,11 +18,14 @@
 package org.openqa.selenium.remote;
 
 import java.io.File;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 /** A file detector that never finds anything. */
+@NullMarked
 public class UselessFileDetector implements FileDetector {
   @Override
-  public File getLocalFile(CharSequence... keys) {
+  public @Nullable File getLocalFile(CharSequence... keys) {
     return null;
   }
 }