Error when scanning metadata files with empty type

[vasily-pozdnyakov]

Description

When scanning METADATA.bzl files, where the field "upstream_type" is an empty string, it gives the following error:
ValueError: Invalid purl: type is a required argument.

How To Reproduce

After fixing #4206 run a scan on METADATA.bzl, where "upstream_type":""
scancode -clup --only-findings --verbose --license-text --license-references --json res.json METADATA.bzl

System configuration

For bug reports, it really helps us to know:

Run on Linux/Windows, Scancode 32.3.2

[vasily-pozdnyakov]

Waiting for #4194 to be merged, since my changes touch the same file.

[pombredanne]

Thanks for the report and PR!