DocumentNamespace is missing in spdx output

[alpianon]

Description

Please leave a brief description of the bug or feature request:

DocumentNamespace is missing in spdx output, while it is mandatory according to spdx spec. Spdx files without DocumentNamespace cannot be validated by spdx-tools

How To Reproduce

Tell us how to reproduce the issue.

• Perform a scan with --spdx-tv outfile.spdx option
• Try to validate the spdx output file with java -jar spdx-tools-2.2.4-jar-with-dependencies.jar Verify outfile.spdx or to convert it to RDF with java -jar spdx-tools-2.2.4-jar-with-dependencies.jar TagToRdf outfile.spdx outfile.rdf
• You get the error The SPDX Document Namespace must be set before other SPDX document properties are set.

System configuration

For bug reports, it really helps us to know:

• What OS are you running on? Linux
• What version of scancode-toolkit was used to generate the scan file? 3.2.3
• What installation method was used to install/run scancode? pip