mbedgt-network-wifi WIFI_CONNECT - check for empty passwd

[VeijoPesonen]

Description

Providing empty password for unsecure Wifi connection should be fine from test case point of view

Pull request type

[ ] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[X] Test update
[ ] Breaking change

[VeijoPesonen]

@ARMmbed/mbed-os-ipcore please review

[cmonr]

+1 for testing difference between an empty string and NULL

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3400
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8469/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3030
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8469/

[mbed-ci]

Test : SUCCESS

Build number : 3200
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8469/3200