Conversion from CycloneDX to SPDX creates invalid SPDX

SPDX JSON file: https://raw.githubusercontent.com/OpenChain-Project/Telco-WG/refs/heads/main/tools/openchain_telco_sbom_validator/open-source-compliance-artifacts/openchain-telco-sbom-validator-0.3.1.spdx.json

```
cyclonedx --version
0.28.2+37262579cb974d91c949f394ba5811a5f8e11202
```

Convert from SPDX to CycloneDX:
```
cyclonedx convert --input-file openchain-telco-sbom-validator-0.3.1.spdx.json --input-format spdxjson --output-file openchain-telco-sbom-validator-cyclonedx-0.3.1.cdx.json --output-format json
```

Result is valid CycloneDX 1.6:
```
cyclonedx validate --input-file openchain-telco-sbom-validator-cyclonedx-0.3.1.cdx.json
BOM validated successfully.
```

Converting back to SPDX:
```
cyclonedx convert --input-file openchain-telco-sbom-validator-cyclonedx-0.3.1.cdx.json --input-format json --output-file openchain-telco-sbom-validator-cyclonedx-0.3.1.cdx-to-spdx.spdx.json --output-format spdxjson
```

Result is invalid SPDX:
```
pyspdxtools -i openchain-telco-sbom-validator-cyclonedx-0.3.1.cdx-to-spdx.spdx.json
ERROR:root:The document is invalid. The following issues have been found:
license_info_from_files must be None if files_analyzed is False, but is: [NOASSERTION]
license_info_from_files must be None if files_analyzed is False, but is: [NOASSERTION]
license_info_from_files must be None if files_analyzed is False, but is: [NOASSERTION]
license_info_from_files must be None if files_analyzed is False, but is: [NOASSERTION]
license_info_from_files must be None if files_analyzed is False, but is: [NOASSERTION]
license_info_from_files must be None if files_analyzed is False, but is: [NOASSERTION]
license_info_from_files must be None if files_analyzed is False, but is: [NOASSERTION]
there must be at least one relationship "SPDXRef-DOCUMENT DESCRIBES ..." or "... DESCRIBED_BY SPDXRef-DOCUMENT" when there is not only a single package present
```

What we lost in the conversion:
```
"creators": [
"Organization: Nokia",
"Tool: Nokia Compliance Tool - 1.0"
],
```
becomes
```
"creators": [
"Tool: Nokia Compliance Tool - 1.0"
],
```

We lose supplier and originator:
```
"originator": "Organization: Nokia",
"supplier": "Organization: https://pypi.org",
```
becomes
```
"originator": "NOASSERTION",
"supplier": "NOASSERTION",
```

There is a useless
```
"licenseInfoFromFiles": [
"NOASSERTION"
],
```
that renders the code invalid.

Also, all relationships are lost, which also renders the code invalid.