FileBasedSink writing to incorrect path when path prefixe has no file component in path (e.g. /tmp/)

This was reported by a user on dev@, original report:
https://lists.apache.org/thread.html/378da40ca7d13e226ca793d2a27af047f9a562f273f6eaa5d677dc4b@%3Cdev.beam.apache.org%3E

This pipeline (where WindowedFilenamePolicy is the one found in org.apache.beam.sdk.io.AvroIOTest) produces files in the wrong directory:
```

stringsPCollection.apply(Window.into(FixedWindows.of(Duration.standardSeconds(5))))

.apply("WritingToOutput",
TextIO.write().withWindowedWrites().withFilenamePolicy(new
WindowedFilenamePolicy("my_pref")).to("/tmp/").withNumShards(1));

```

I expected the output files to be written to /tmp/ directory but they are not. They are written to root directory which is unexpected behavior.

I think the problem is org.apache.beam.sdk.io.FileBasedSink.ExtractDirectory ...
This main method shows the problem:
```

import org.apache.beam.sdk.io.FileBasedSink;
import org.apache.beam.sdk.io.fs.ResourceId;
import org.apache.beam.sdk.options.ValueProvider;
import
org.apache.beam.sdk.options.ValueProvider.StaticValueProvider;

public class Test {

public static
void main(String[] args) {
ResourceId res = FileBasedSink.convertToFileResourceIfPossible("/tmp/");

System.out.println("Resource is " + res + ", current directory is " +
res.getCurrentDirectory() +
", filename is " + res.getFilename());
FileBasedSink mockFBS = new
FileBasedSink(StaticValueProvider.of(res),
null) {

@Override
public org.apache.beam.sdk.io.FileBasedSink.WriteOperation
createWriteOperation()
{
return null;
}
};

final ValueProvider provider =
mockFBS.getBaseOutputDirectoryProvider();

System.out.println("BaseOutputProvider is " + provider + ",
isAccessible=" + provider.isAccessible()
+ ", getValue=" + provider.get());
}

}

```

The output is
```

Resource is /tmp, current directory is //, filename is tmp
BaseOutputProvider is
NestedValueProvider{value=StaticValueProvider{value=/tmp}},
isAccessible=true,
getValue=//

```

Imported from Jira [BEAM-2448](https://issues.apache.org/jira/browse/BEAM-2448). Original Jira may contain additional context.
Reported by: lcwik.

Start with org.apache.beam.sdk.io.FileBasedSink, especially ExtractDirectory, and reproduce the behavior using the Java example in the issue with the /tmp/ path prefix. Done means the base output directory resolves to /tmp/ rather than the root directory when the prefix has no file component.

Written by the indexing model from the issue text.