check-meta: defaultCPEParts: accomodate null pname

[ShamrockLee]

pname and version are sometime specified as null to get them ignored. If we only check the existence of pname without checking if it's null, we'll end up specifying meta.cpeParts.product = null.

This PR unbreaks

• buildEnv: support fixed-point arguments #432957

bundlerEnv passes pname and version as null to buildEnv in case they pname and version isn't specified when calling bundlerEnv. In previous implementation, buildEnv only passes pname and version down if they are non-null. To simplify name handling, PR #432957 passes name, pname and version as specified to stdenvNoCC.mkDerivation and surface the issue.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[ShamrockLee]

nixpkgs-review result

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Command: nixpkgs-review pr 458529
Commit: 5ffd35a7064449f2aabd5aef0d681b06b3022187

[MattSturgeon]

Backport failed for release-25.05, because it was unable to cherry-pick the commit(s).

Looks like CPE field itself wasn't backported, so this doesn't need backporting after all: #439074 (comment)

There's also discussion around postponing until after 25.11: #439074 (comment)