unit-rule: Fix panic on grafonnet generated dashboards

[v-zhuravlev]

Example:

panic: interface conversion: interface {} is []interface {}, not []dashboard.ValueMapOrRangeMapOrRegexMapOrSpecialValueMap

goroutine 1 [running]:
github.com/grafana/dashboard-linter/lint.getValueMappings(...)
        /home/code/go/pkg/mod/github.com/grafana/[email protected]/lint/rule_panel_units.go:136
github.com/grafana/dashboard-linter/lint.NewPanelUnitsRule.func1({{0x0, 0x0, 0x0}, {0xc00039e490, 0x10}, {{0xc00022c2c8, 0x3, 0x4}}, {{0x0, 0x0, ...}}, ...}, ...)
        /home/code/go/pkg/mod/github.com/grafana/[email protected]/lint/rule_panel_units.go:92 +0x6a5
github.com/grafana/dashboard-linter/lint.PanelRuleFunc.Lint({{0x1500a72, 0x10}, {0x1538a1c, 0x34}, 0xc0001130a0}, {{0x0, 0x0, 0x0}, {0xc00039e490, 0x10}, ...}, ...)
        /home/code/go/pkg/mod/github.com/grafana/[email protected]/lint/rules.go:69 +0x276
github.com/grafana/dashboard-linter/lint.(*RuleSet).Lint(0xc0001299c0, {0xc000129a00, 0x1, 0xc00039e490?})
        /home/code/go/pkg/mod/github.com/grafana/[email protected]/lint/rules.go:209 +0x127
main.init.func2(0xc0000ed700?, {0xc000543670?, 0x1?, 0x14f1b27?})
        /home/code/go/pkg/mod/github.com/grafana/[email protected]/main.go:73 +0x510
github.com/spf13/cobra.(*Command).execute(0x2125820, {0xc000543640, 0x1, 0x1})
        /home/code/go/pkg/mod/github.com/spf13/[email protected]/command.go:985 +0xaaa
github.com/spf13/cobra.(*Command).ExecuteC(0x2125de0)
        /home/code/go/pkg/mod/github.com/spf13/[email protected]/command.go:1117 +0x3ff
github.com/spf13/cobra.(*Command).Execute(...)
        /home/code/go/pkg/mod/github.com/spf13/[email protected]/command.go:1041
main.main()
        /home/code/go/pkg/mod/github.com/grafana/[email protected]/main.go:176 +0x1a

when empty mapping is provided:

            "fieldConfig": {
               "overrides": [
                  {
                     "matcher": {
                        "id": "byName",
                        "options": "Topic start offset"
                     },
                     "properties": [
                        {
                           "id": "mappings",
                           "value": [ ]
                        },
                        {
                           "id": "unit",
                           "value": "none"
                        }
                     ]
                  },
...

[rgeyer]

Had to go to the whole file to grok the change here, I was worried you were skipping the error case ;}

Nice simplification, thank you!