fix(grpc): return error if credentials are detected to be null (#2142)

Author: BenWhitehead

google-cloud-storage/src/main/java/com/google/cloud/storage/GrpcStorageOptions.java

@@ -48,6 +48,7 @@
 import com.google.cloud.storage.UnifiedOpts.UserProject;
 import com.google.cloud.storage.spi.StorageRpcFactory;
 import com.google.common.base.MoreObjects;
+import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import com.google.storage.v2.ReadObjectRequest;
@@ -169,6 +170,7 @@ private Tuple<StorageSettings, Opts<UserProject>> resolveSettingsAndOpts() throw
 
     Opts<UserProject> defaultOpts = Opts.empty();
     CredentialsProvider credentialsProvider;
+    Preconditions.checkState(credentials != null, "Unable to resolve credentials");
     if (credentials instanceof NoCredentials) {
       credentialsProvider = NoCredentialsProvider.create();
     } else {