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 #7543

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7544)

Configure

@@ -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 $d = dirname($_);