fix: run config validation for falsy value

0 is considered as falsy but it's still a valid config

lib/config.js

@@ -155,7 +155,7 @@ class Config {
     if (shouldActiveRule(config[rule.name])) {
       let ruleConfig = extractRuleConfig(config[rule.name], rule.name);
       this.activatedRules[rule.name] = rule;
-      if (ruleConfig) {
+      if (ruleConfig !== null && ruleConfig !== undefined) {
         ruleConfig = rule.configTransform ? rule.configTransform(ruleConfig) : ruleConfig;
         if (rule.validateConfig) {
           rule.validateConfig(ruleConfig);