nix: Correctly handle commitSha == null in nix devshell (#51319)

Release Notes:

- N/A

nix/build.nix

@@ -224,7 +224,7 @@ let
         };
         ZED_UPDATE_EXPLANATION = "Zed has been installed using Nix. Auto-updates have thus been disabled.";
         RELEASE_VERSION = version;
-        ZED_COMMIT_SHA = commitSha;
+        ZED_COMMIT_SHA = lib.optionalString (commitSha != null) "${commitSha}";
         LK_CUSTOM_WEBRTC = pkgs.callPackage ./livekit-libwebrtc/package.nix { };
         PROTOC = "${protobuf}/bin/protoc";