[IOS] Crash on get_network_instances for devices that do not support show vrf detail

[DavidVentura]

Description of Issue/Question

Calling get_network_instances on a Cisco C2960XR crashes. I expect it not to crash.

Did you follow the steps from https://github.com/napalm-automation/napalm#faq

(Place an x between the square brackets where applicable)

• Yes
• No

Setup

napalm version

napalm==3.3.1

Network operating system version

cisco WS-C2960XR-24TD-I (APM86XXX) processor (revision U0) with 524288K bytes of memory.

Switch Ports Model                     SW Version            SW Image                 
------ ----- -----                     ----------            ----------               
*    1 28    WS-C2960XR-24TD-I         15.2(2)E6             C2960X-UNIVERSALK9-M     

Steps to Reproduce the Issue

Call get_network_instances()

Error Traceback

Traceback (most recent call last):
  File "device.py", line 306, in get_truth_data
    vrfs = self.device.get_network_instances()
  File "venv/lib/python3.8/site-packages/napalm/ios/ios.py", line 3439, in get_network_instances
    vrf_name, RD = re.match(r"^VRF (\S+).*RD (.*);", first_part).groups()
AttributeError: 'NoneType' object has no attribute 'groups'

This is because sh_vrf_detail contains Invalid input detected