fix: remoteaccess connect ssh handling of empty port-forwarding options

[reubenmiller]

Fixing a bug where the remoteaccess port-forwarding option was not ignore empty values (which is the default) and it was resulting in an invalid ssh command.

This resulted in the following command failing as shown below:

$ c8y remoteaccess connect ssh --device rpi5-abcdef --user pi

2025-01-29T09:27:02.994+0100	INFO	Executing command: ssh -o ServerAliveInterval=120 -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -p 59380 root@127.0.0.1 -L :127.0.0.1:

Starting interactive ssh session with 5733465816 (https://myiot.example.cumulocity.com) with port-forwarding :127.0.0.1:

Bad local forwarding specification ':127.0.0.1:'
Duration: 26m