adding deviation for cisco delimiter, and skipping empty string

[fmolinar]

Adding the deviation for the banner test as Cisco devices expect a delimiter to be sent, and skipping the empty string banner test since Cisco currently doesnt support empty string send as banner.

[OpenConfigBot]

Pull Request Functional Test Report for #3756 / 1a32a6e

Virtual Devices

Device	Test	Test Documentation	Job	Raw Log
Arista cEOS		System-1: System testing
Cisco 8000E		System-1: System testing
Cisco XRd		System-1: System testing
Juniper ncPTX		System-1: System testing
Nokia SR Linux		System-1: System testing
Openconfig Lemming		System-1: System testing

Hardware Devices

Device	Test	Test Documentation	Raw Log
Arista 7808		System-1: System testing
Cisco 8808		System-1: System testing
Juniper PTX10008		System-1: System testing
Nokia 7250 IXR-10e		System-1: System testing

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