Error when scanning metadata files with empty type

### 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