kded: Fix crash when connected output reports no modes

When using a KVM switch with EDID emulation that supports two (or more) screens connected, some switches will report all outputs as connected, even if only one monitor is plugged into the switch. In that case, the switch reports one of the outputs as connected but with no available modes. As a result, the KDED code crashed as it tried to find the best mode and dereference it without checking when initializing the output. Since output without any modes cannot be enabled, we detect that and explicitly disable the output instead.