fix: prevent crash when checking if a missing file exists googleapis#856

Fixes a crash that occurred when autoDetectRequesterPays is set and
a Files.exists() call is made on a file that doens't exist.

Refs: googleapis#856

google-cloud-nio/src/main/java/com/google/cloud/storage/contrib/nio/CloudStorageConfiguration.java

@@ -123,7 +123,7 @@ public static final class Builder {
     private int blockSize = CloudStorageFileSystem.BLOCK_SIZE_DEFAULT;
     private int maxChannelReopens = 0;
     private @Nullable String userProject = null;
-    // This of this as "clear userProject if not RequesterPays"
+    // Think of this as "clear userProject if not RequesterPays"
     private boolean useUserProjectOnlyForRequesterPaysBuckets = false;
     private ImmutableList<Integer> retryableHttpCodes = ImmutableList.of(500, 502, 503);
     private ImmutableList<Class<? extends Exception>> reopenableExceptions =

google-cloud-nio/src/main/java/com/google/cloud/storage/contrib/nio/CloudStorageFileSystemProvider.java

@@ -987,7 +987,7 @@ public boolean requesterPays(String bucketName) {
    * requester-pays.
    */
   public CloudStorageFileSystemProvider withNoUserProject() {
-    return new CloudStorageFileSystemProvider("", this.storageOptions);
+    return new CloudStorageFileSystemProvider(null, this.storageOptions);
   }
 
   /** Returns the project that is assigned to this provider. */

google-cloud-nio/src/main/java/com/google/cloud/storage/contrib/nio/CloudStoragePath.java

@@ -23,6 +23,7 @@
 import com.google.cloud.storage.Blob;
 import com.google.cloud.storage.BlobId;
 import com.google.cloud.storage.Storage;
+import com.google.common.base.Strings;
 import com.google.common.collect.UnmodifiableIterator;
 import java.io.File;
 import java.net.URI;
@@ -113,7 +114,7 @@ boolean seemsLikeADirectoryAndUsePseudoDirectories(Storage storage) {
     }
     String userProject = fileSystem.config().userProject();
     Page<Blob> list = null;
-    if (userProject != null) {
+    if (!Strings.isNullOrEmpty(userProject)) {
       list =
           storage.list(
               this.bucket(),

google-cloud-nio/src/test/java/com/google/cloud/storage/contrib/nio/it/ITGcsNio.java

@@ -327,6 +327,18 @@ public void testAutodetectWhenNotRequesterPays() throws IOException {
         "");
   }
 
+  @Test
+  public void testFilesExistDoesntCrashWhenRequesterPays() throws IOException {
+    CloudStorageConfiguration config =
+        CloudStorageConfiguration.builder()
+            .autoDetectRequesterPays(true)
+            .userProject(project)
+            .build();
+    CloudStorageFileSystem testBucket =
+        CloudStorageFileSystem.forBucket(BUCKET, config, storageOptions);
+    Assert.assertFalse(Files.exists(testBucket.getPath("path")));
+  }
+
   @Test
   public void testAutoDetectNoUserProject() throws IOException {
     CloudStorageFileSystem testBucket = getRequesterPaysBucket(false, "");