fix: Prevent NPE when bucket doesn't exist #857 (#860)

Fixes a NullPointerException when creating a Path object with a bucket
that doesn't exist. This only occurred when autoDetectRequesterPays = true.

Refs: #857

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Fixes #857☕️

If you write sample code, please follow the [samples format](
https://github.com/GoogleCloudPlatform/java-docs-samples/blob/main/SAMPLE_FORMAT.md).

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

@@ -963,8 +963,13 @@ public String toString() {
   public boolean requesterPays(String bucketName) {
     initStorage();
     try {
-      // instead of true/false, this method returns true/null.
-      Boolean isRP = storage.get(bucketName).requesterPays();
+      final Bucket bucket = storage.get(bucketName);
+      // If the bucket doesn't exist it can't be requester pays.
+      if (bucket == null) {
+        return false;
+      }
+      // instead of true/false, this method returns true/null
+      Boolean isRP = bucket.requesterPays();
       return isRP != null && isRP.booleanValue();
     } catch (StorageException ex) {
       if ("userProjectMissing".equals(ex.getReason())) {

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

@@ -328,7 +328,26 @@ public void testAutodetectWhenNotRequesterPays() throws IOException {
   }
 
   @Test
-  public void testFilesExistDoesntCrashWhenRequesterPays() throws IOException {
+  public void testRequesterPaysOnNonexistentBucket() {
+    CloudStorageConfiguration config =
+        CloudStorageConfiguration.builder()
+            .autoDetectRequesterPays(true)
+            .userProject(project)
+            .usePseudoDirectories(true)
+            .build();
+
+    final String bucketThatDoesntExist = "abuckethatdoesntexist";
+    final String subPath = "hello";
+    CloudStorageFileSystem testBucket =
+        CloudStorageFileSystem.forBucket(bucketThatDoesntExist, config, storageOptions);
+    final CloudStoragePath aPathThatDoesntExist = testBucket.getPath(subPath);
+    Assert.assertEquals(
+        aPathThatDoesntExist.toUri().toString(), "gs://" + bucketThatDoesntExist + "/" + subPath);
+    Assert.assertFalse(testBucket.provider().requesterPays(bucketThatDoesntExist));
+  }
+
+  @Test
+  public void testFilesExistBehaviorRequesterPays() {
     CloudStorageConfiguration config =
         CloudStorageConfiguration.builder()
             .autoDetectRequesterPays(true)