3.6.0: enable_v2_engine_router: true leads to panic

[Nachtfalkeaw]

Describe the bug

querier:
  engine_v2:
    enable: true
query_range:
  enable_v2_engine_router: true

leads to panic after start

Nov 19 01:22:42 hostname systemd[1]: Started Grafana loki.
Nov 19 01:22:42 hostname loki[971719]: panic: v2EngineHandler cannot be nil
Nov 19 01:22:42 hostname loki[971719]: goroutine 1 [running]:
Nov 19 01:22:42 hostname loki[971719]: github.com/grafana/loki/v3/pkg/querier/queryrange.newEngineRouterMiddleware(...)
Nov 19 01:22:42 hostname loki[971719]: #011/src/loki/pkg/querier/queryrange/engine_router.go:48
Nov 19 01:22:42 hostname loki[971719]: github.com/grafana/loki/v3/pkg/querier/queryrange.NewMetricTripperware.func3({0x5456d20, 0xc000ee78c8})
Nov 19 01:22:42 hostname loki[971719]: #011/src/loki/pkg/querier/queryrange/roundtrip.go:1088 +0x14f2
Nov 19 01:22:42 hostname loki[971719]: github.com/grafana/loki/v3/pkg/querier/queryrange/queryrangebase.MiddlewareFunc.Wrap(0xc000dd17a0?, {0x5456d20?, 0xc000ee78c8?})
Nov 19 01:22:42 hostname loki[971719]: #011/src/loki/pkg/querier/queryrange/queryrangebase/roundtrip.go:93 +0x26
Nov 19 01:22:42 hostname loki[971719]: github.com/grafana/loki/v3/pkg/querier/queryrange.NewMiddleware.func1({0x5456d20, 0xc000ee78c8})
Nov 19 01:22:42 hostname loki[971719]: #011/src/loki/pkg/querier/queryrange/roundtrip.go:296 +0x170
Nov 19 01:22:42 hostname loki[971719]: github.com/grafana/loki/v3/pkg/querier/queryrange/queryrangebase.MiddlewareFunc.Wrap(0xc001e87a90?, {0x5456d20?, 0xc000ee78c8?})
Nov 19 01:22:42 hostname loki[971719]: #011/src/loki/pkg/querier/queryrange/queryrangebase/roundtrip.go:93 +0x26
Nov 19 01:22:42 hostname loki[971719]: github.com/grafana/loki/v3/pkg/loki.(*Loki).initQueryFrontend(0xc000718000)
Nov 19 01:22:42 hostname loki[971719]: #011/src/loki/pkg/loki/modules.go:1255 +0x7e3
Nov 19 01:22:42 hostname loki[971719]: github.com/grafana/dskit/modules.(*Manager).initModule(0xc0000ea618, {0xc0010bb038, 0x3}, 0xc000dd2558, 0xc0010f40c0)
Nov 19 01:22:42 hostname loki[971719]: #011/src/loki/vendor/github.com/grafana/dskit/modules/modules.go:136 +0x1ea
Nov 19 01:22:42 hostname loki[971719]: github.com/grafana/dskit/modules.(*Manager).InitModuleServices(0xc0000ea618, {0xc0010c9f40, 0x2, 0x8c1b9c?})
Nov 19 01:22:42 hostname loki[971719]: #011/src/loki/vendor/github.com/grafana/dskit/modules/modules.go:108 +0xf4
Nov 19 01:22:42 hostname loki[971719]: github.com/grafana/loki/v3/pkg/loki.(*Loki).Run(0xc000718000, {0x0?, {0x4?, 0x2?, 0x7fc6c80?}})
Nov 19 01:22:42 hostname loki[971719]: #011/src/loki/pkg/loki/loki.go:549 +0x91
Nov 19 01:22:42 hostname loki[971719]: main.main()
Nov 19 01:22:42 hostname loki[971719]: #011/src/loki/cmd/loki/main.go:130 +0x149d
Nov 19 01:22:42 hostname systemd[1]: loki.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
Nov 19 01:22:42 hostname systemd[1]: loki.service: Failed with result 'exit-code'.

To Reproduce
Steps to reproduce the behavior:

1. enable_v2_engine_router: true
2. Start loki 3.6.0 monolithic

Expected behavior
loki will start

Environment:

• Infrastructure: monolithic
• Deployment tool: binary

Screenshots, Promtail config, or terminal output
see above