Treat the main target as empty if it's not provided together with the SBOM.

**Description**
We allow the user to run the analysis for the dependencies listed in the SBOM file (provided via `-sbom`) but the main target software component doesn't have a repository available.
For example:
```bash
macaron analyze -sbom <path_to_sbom> -purl <no_repo_component_purl> 
```