do not generate apt sources.list entry if SUITE is empty

coverage.txt

@@ -433,3 +433,5 @@ Modes: unshare
 
 Test: zombie-reaping
 Modes: unshare
+
+Test: empty-suite

mmdebstrap

@@ -5265,6 +5265,10 @@ sub get_sourceslist_by_suite {
     my $compstr         = shift;
     my $suite_by_vendor = shift;
 
+    if (!$suite) {
+        error "get_sourceslist_by_suite cannot operate with an empty suite";
+    }
+
     my @debstable    = keys %{ $suite_by_vendor->{'debian'} };
     my @ubuntustable = keys %{ $suite_by_vendor->{'ubuntu'} };
     my @tanglustable = keys %{ $suite_by_vendor->{'tanglu'} };
@@ -6525,6 +6529,10 @@ sub main() {
                               };
                         }
                     } elsif ($arg =~ /:\/\//) {
+                        if (!$options->{suite}) {
+                            error(  "cannot create sources.list entry for URI"
+                                  . "with empty suite");
+                        }
                         my $content = join ' ',
                           (
                             "deb$signedby",
@@ -6580,6 +6588,9 @@ sub main() {
                         error "invalid mirror: $arg";
                     }
                 }
+            } elsif (!length($options->{suite})) {
+                warning
+                  "cannot guess apt sources.list entry with empty suite name";
             } else {
                 my $sourceslist
                   = get_sourceslist_by_suite($options->{suite},

tests/empty-suite

@@ -0,0 +1,13 @@
+#!/bin/sh
+set -eu
+export LC_ALL=C.UTF-8
+export SOURCE_DATE_EPOCH={{ SOURCE_DATE_EPOCH }}
+
+trap "rm -f /tmp/debian-chroot.tar" EXIT INT TERM
+
+{{ CMD }} --variant={{ VARIANT }} --verbose \
+  --setup-hook='echo deb {{ MIRROR }} {{ DIST }} main >> "$1"/etc/apt/sources.list' \
+  '' /tmp/debian-chroot.tar
+
+cmp ./cache/mmdebstrap-{{ DIST }}-{{ VARIANT }}.tar /tmp/debian-chroot.tar \
+  || diffoscope ./cache/mmdebstrap-{{ DIST }}-{{ VARIANT }}.tar /tmp/debian-chroot.tar