Motivation for this change

Not all dependencies mentioned in a pom.xml necessarily have JARs or other build artifacts; some may be POMs (metadata) only. In this case, metadata.url may be set in a dependencies entry (inside the project-info.json file created by mvn2nix and consumed by buildMaven) where url is not present.

NixOS/mvn2nix-maven-plugin#17 contains an example, with notes on how to reproduce a failure (wherein mvn2nix-maven-plugin generates entries with no url for artifact-free POMs, and buildMaven subsequently fails).

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests) None applicable
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip" "Nothing changed"
• Tested execution of all binary files (usually in ./result/bin/) N/A
• Determined the impact on package closure size (by running nix path-info -S before and after) N/A
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.