ISIS: T6332: Fix isis not working only ipv6 (backport #3537)

[mergify]

ISIS is not working on a IPv6 only environment , needs to be enable the topology when is not ipv4 unicast

Change Summary

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

https://vyos.dev/T6332

Related PR(s)

Component(s) name

isis

Proposed changes

How to test

#vyos without topology ipv6 , don't see the routes 

set interfaces dummy dum0 address '2001:db8:1:1::1/64'

set interfaces ethernet eth1 address '2001:db8:3::1/64'
set interfaces ethernet eth1 mtu '1500'
set protocols isis dynamic-hostname
set protocols isis interface dum0
set protocols isis interface eth1 network point-to-point
set protocols isis level 'level-2'
set protocols isis net '49.0001.0bad.cafe.0001.0

# vyos new command: 

set protocols isis topology 'ipv6-unicast'

Smoketest result

ok
test_isis_06_spf_delay_bfd (__main__.TestProtocolsISIS.test_isis_06_spf_delay_bfd) ...
All types of spf-delay must be configured. Missing: short-delay, long-
delay, init-delay, time-to-learn


All types of spf-delay must be configured. Missing: short-delay, long-
delay, time-to-learn


All types of spf-delay must be configured. Missing: short-delay, time-
to-learn


All types of spf-delay must be configured. Missing: time-to-learn

ok
test_isis_07_segment_routing_configuration (__main__.TestProtocolsISIS.test_isis_07_segment_routing_configuration) ... ok
test_isis_08_ldp_sync (__main__.TestProtocolsISIS.test_isis_08_ldp_sync) ... ok
test_isis_09_lfa (__main__.TestProtocolsISIS.test_isis_09_lfa) ... ok
test_isis_10_topology (__main__.TestProtocolsISIS.test_isis_10_topology) ... ok

----------------------------------------------------------------------
Ran 9 tests in 68.197s

OK

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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This is an automatic backport of pull request #3537 done by [Mergify](https://mergify.com).