dotnetCorePackages.combinePackages: fix bug introduced in #187118

#         3. Something else?

let cli = builtins.head packages;

in

assert lib.assertMsg ((builtins.length packages) < 1)

assert lib.assertMsg ((builtins.length packages) > 0)

    ''You must include at least one package, e.g

      `with dotnetCorePackages; combinePackages [

          sdk_3_1 aspnetcore_5_0