Revert "Don't crash on a missing Subject in index.txt"

This reverts commit 1e05c6d0.

Empty subjects should be permissible.
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5444)

apps/ca.c

@@ -664,10 +664,6 @@ end_of_options:
                 goto end;
             }
         }
-        if (pp[DB_name][0] == '\0') {
-            BIO_printf(bio_err, "entry %d: bad Subject\n", i + 1);
-            goto end;
-        }
     }
     if (verbose) {
         TXT_DB_write(bio_out, db->db);