return immediately if nothing to do in download, extract and essential stages

mmdebstrap

@@ -1696,6 +1696,11 @@ sub run_download() {
     # (essential variant) then we have to compute the package set ourselves.
     # Same if we want to install priority based variants.
     if (any { $_ eq $options->{variant} } ('extract', 'custom')) {
+        if (scalar @pkgs_to_install == 0) {
+            info "nothing to download -- skipping...";
+            return ([], []);
+        }
+
         if ($options->{dryrun}) {
             info "simulate downloading packages with apt...";
         } else {
@@ -1902,6 +1907,11 @@ sub run_extract() {
         return;
     }
 
+    if (scalar @{$essential_pkgs} == 0) {
+        info "nothing to extract -- skipping...";
+        return;
+    }
+
     info "extracting archives...";
     print_progress 0.0;
     my $counter = 0;
@@ -2137,6 +2147,11 @@ sub run_essential() {
     my $essential_pkgs = shift;
     my $chrootcmd      = shift;
 
+    if (scalar @{$essential_pkgs} == 0) {
+        info "no essential packages -- skipping...";
+        return;
+    }
+
     if ($options->{mode} eq 'chrootless') {
         if ($options->{dryrun}) {
             info "simulate installing packages...";