frontend: do not run `packages`-related code on the absence of packages

[cirocosta]

 => => sha256:7413c47ba209e555018c4be91101d017737f24b0c9d1f65339b97a4da98acb2a 26.69MB / 26.69MB                                                                                                                                                         0.0s
 => => unpacking docker.io/concourse/concourse:latest@sha256:38cb311fac63c09dee250c888c7488b1aba964404bdcc268e02ad916af509bc9                                                                                                                            5.2s
 => local://bin                                                                                                                                                                                                                                          0.8s
 => => transferring bin: 29.21MB                                                                                                                                                                                                                         0.8s
 => /usr/local/bin/estaleiro apt-repositories --output=/keys.yml                                                                                                                                                                                        10.3s
 => /usr/local/bin/estaleiro collect --input=/var/lib/dpkg/status --output=/bom/initial-packages.yml                                                                                                                                                     0.6s
 => /usr/local/bin/estaleiro base --output=/bom/base.yml                                                                                                                                                                                                 0.5s
 => ERROR /usr/local/bin/estaleiro apt-packages --output=/pkgs.yml --debs=/var/lib/estaleiro/debs                                                                                                                                                        0.5s
 => copy /keys.yml /keys.yml                                                                                                                                                                                                                             0.1s
------
 > /usr/local/bin/estaleiro apt-packages --output=/pkgs.yml --debs=/var/lib/estaleiro/debs:
#4 0.327 the required flag `-p' was not specified
------
error: failed to solve: rpc error: code = Unknown desc = failed to build LLB: executor failed running [/usr/local/bin/estaleiro apt-packages --output=/pkgs.yml --debs=/var/lib/estaleiro/debs]: exit code: 1

sample:

# syntax=cirocosta/estaleiro

# an example showing how leveraging `estaleiro` as just a final step would look
# like.
#
image "concourse" {
  base_image {
    name = "concourse/concourse"
  }
}

[cirocosta]

👍

[cirocosta]

still, that validate() step is going to be very necessary