promtool: generate rulesLintConfig when --lint=none is set (#17414)

* Correctly generate rulesLintConfig when --lint=none is set (#17399)

If I run promtool check config --lint=none I get:

```
Checking rules.yml
  FAILED:
rules.yml: unset nameValidationScheme
```

This is because passing --lint=none stops newConfigLintConfig from generating rulesLintConfig which is needed for validation.
It means that defaults are used then, one of which is unset value for metric name validation, causing this error.
Fix this by handling --lint=none case correctly and still generating rulesLintConfig.

---------

Signed-off-by: Lukasz Mierzwa <l.mierzwa@gmail.com>
Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>
Co-authored-by: Łukasz Mierzwa <l.mierzwa@gmail.com>

cmd/promtool/main.go

@@ -476,13 +476,15 @@ type rulesLintConfig struct {
 }
 
 func newRulesLintConfig(stringVal string, fatal, ignoreUnknownFields bool, nameValidationScheme model.ValidationScheme) rulesLintConfig {
-	items := strings.Split(stringVal, ",")
 	ls := rulesLintConfig{
 		fatal:                fatal,
 		ignoreUnknownFields:  ignoreUnknownFields,
 		nameValidationScheme: nameValidationScheme,
 	}
-	for _, setting := range items {
+	if stringVal == "" {
+		return ls
+	}
+	for _, setting := range strings.Split(stringVal, ",") {
 		switch setting {
 		case lintOptionAll:
 			ls.all = true
@@ -534,9 +536,7 @@ func newConfigLintConfig(optionsStr string, fatal, ignoreUnknownFields bool, nam
 		rulesOptions = nil
 	}
 
-	if len(rulesOptions) > 0 {
-		c.rulesLintConfig = newRulesLintConfig(strings.Join(rulesOptions, ","), fatal, ignoreUnknownFields, nameValidationScheme)
-	}
+	c.rulesLintConfig = newRulesLintConfig(strings.Join(rulesOptions, ","), fatal, ignoreUnknownFields, nameValidationScheme)
 
 	return c
 }

cmd/promtool/testdata/prometheus-config.lint.too_long_scrape_interval.yml

@@ -1,3 +1,5 @@
 scrape_configs:
   - job_name: too_long_scrape_interval_test
     scrape_interval: 10m
+rule_files:
+  - prometheus-config.rules.good.yml

cmd/promtool/testdata/prometheus-config.rules.good.yml

@@ -0,0 +1,3 @@
+groups:
+  - name: rules
+    rules: []