bgpd: correction in json output structure for no data case

Problem:

The VTYSH JSON output is not in the proper format when bgp l2vpn-evpn
information is missing or not populated for a given vni-id. This results
in a malformed JSON structure, which causes a parsing error.

Fix:
Corrected the JSON output structure generated during command execution
to properly handle cases where no bgp l2vpn-evpn data is available for a
given vni-id.

Before Fix:

```
{}
{
}
```

After Fix:

```
{
}

```

Signed-off-by: Sindhu Parvathi Gopinathan's <sgopinathan@nvidia.com>
(cherry picked from commit a8f1785)

Author: Sindhu Parvathi Gopinathan

bgpd/bgp_evpn_vty.c

@@ -3331,13 +3331,9 @@ static void evpn_show_vni(struct vty *vty, struct bgp *bgp, vni_t vni,
 		}
 	}
 
-	if (!found) {
-		if (json) {
-			vty_out(vty, "{}\n");
-		} else {
-			vty_out(vty, "VNI not found\n");
-			return;
-		}
+	if (!found && !json) {
+		vty_out(vty, "VNI not found\n");
+		return;
 	}
 }