Missing PackageVerificationCode should be flagged as invalid

[vargenau]

MEV.spdx.txt

pyspdxtools -i MEV.spdx

gives no error, so the SPDX SBOM is valid.

However, as there is no FilesAnalyzed, it defaults to true so PackageVerificationCode is mandatory.

So the SBOM should be flagged as invalid.

See also: https://github.com/spdx/tools-java/issues/188