ftp: don't match unset ftp.dynamic_port

Treat dyn_port 0 as an absent value so numeric comparisons do not
match FTP transactions without a negotiated data port.

Ticket: #8392

src/detect-ftp-dynamic-port.c

@@ -87,6 +87,8 @@ static int DetectFtpDynamicPortMatch(DetectEngineThreadCtx *det_ctx, Flow *f, ui
     FTPTransaction *tx = (FTPTransaction *)txv;
     if (tx->command_descriptor.command_code == FTP_COMMAND_UNKNOWN)
         return 0;
+    if (tx->dyn_port == 0)
+        return 0;
 
     const DetectU16Data *ftpd = (const DetectU16Data *)ctx;