Possible Incorrect RDF Generation

### Description
Creating this issue per the request of @pombredanne

I am attempting to create an RDF File with Scancode and import it into the "spdx-tools" tool. I appear to create the RDF file without issue, but when I attempt to import it into the second tool, I receive the following error:

```
ERROR StatusLogger No log4j2 configuration file found. Using default configuration: logging only errors to the console. Set system property 'log4j2.debug' to show Log4j2 internal initialization logging.
Error creating SPDX Document: No external document reference was found for URI http://www.spdx.org/files#None
```

This appears to be occurring due to the presence of multiple instances of the following:
```
"
```

I have tried generating an RDF file for three separate packages, and have run into this issue twice (third time was an issue related to XML). I have also attempted to generate the file as a '.tag' file and then converting it to an RDF file via 'spdx-tools', but this has not circumvented the issue either. The sample RDF file packaged with 'spdx-tools' also opens properly in 'spdx-tools'.

What I am trying to understand is what exactly these lines allude to and if there are any settings I can configure on the ScanCode side to either omit these lines or properly populate them.

### How To Reproduce

- **Scancode Command**:
```
scancode -clipeu -n 2 --verbose --spdx-rdf freetds-verbose.rdf freetds-1.1.24
```

- **SPDX-Tools Comman
```
java -jar /home/jbarry/Downloads/spdx-tools-2.2.1-jar-with-dependencies.jar SPDXViewer freetds-verbose.rdf
```

### System configuration
RHEL 7.8
Scancode-toolkit Version 3.1.1
SPDX Java tools Version 2.2.1
Python 3.6.8

I have attached a Zip file containing the verbose output of ScanCode, the RDF file itself, and the verbose output of SPDX-Tools. Please let me know if any additional details are desired at this time.

Thank you.

[freetds_scancode_files.zip](https://github.com/nexB/scancode-toolkit/files/4913484/freetds_scancode_files.zip)