WAN WiFi Client should allow zero length passphrase

[ab7pa]

The WAN WiFi Client section should also allow a zero-length passphrase. Legacy code had this test:
if( "$wifi3_enable" eq "1" and (length( $wifi3_key ) < 8 or length($wifi3_key) > 64) and ! length($wifi3_key) == 0)

So the new Lua code should allow a zero length passphrase too:
line 685: if #wifi3_key < 8 or #wifi3_key > 64 then
(the test should include #wifi3_key ~= 0 )