Allow an empty NPN/ALPN protocol list in the tests

Allow ourselves to configure an empty NPN/ALPN protocol list and test what
happens if we do.

Follow on from CVE-2024-5535

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from openssl#24718)

(cherry picked from commit c54e56f)

test/handshake_helper.c

@@ -341,6 +341,12 @@ static int parse_protos(const char *protos, unsigned char **out, size_t *outlen)
 
     len = strlen(protos);
 
+    if (len == 0) {
+        *out = NULL;
+        *outlen = 0;
+        return 1;
+    }
+
     /* Should never have reuse. */
     if (!TEST_ptr_null(*out)
             /* Test values are small, so we omit length limit checks. */