Fix the following issues mentioned in Pixel SBOM review.

1) PackageSupplier should be NOASSERTION if there is no homepage
   information in METADATA file of source packages
2) PackageDownloadLocation of upstream packages should be NOASSERTION if
   there is no code repository URL in METADATA file of source packages

Test: CIs
Test: atest --host sbom_writers_test
Change-Id: I8a0298b7bacc2f96555f9d7dde0d21ada8c6b564