Configure: ensure empty arrays aren't created inadvertently
Just refering to a hash table element as an array reference will
automatically create that element.  Avoid that by defaulting to
a separate empty array reference.
Fixes
@@ -2344,7 +2344,7 @@ EOF
my %dirs = ();
my $pd = dirname($product);
- foreach (@{$unified_info{sources}->{$product}},
+ foreach (@{$unified_info{sources}->{$product} // []},
@{$unified_info{shared_sources}->{$product} // []}) {
my $d = dirname($_);
