luci-mod-network: fix SAE/WPA check on network join

The method array may be null in case of an open network, also not all
occurrences of `is_sae` and `is_psk` were properly checked.

Resolve the issue by moving the length check to the variable initialization.

Fixes: ba98a2fd

 ("luci-mod-network: fix logic bug in parse enc for network join")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>