fix: relax nullable rule checks

as the result of disscussion at:
https://github.com/xeptore/commitiquette/pull/1#pullrequestreview-626057744,
we decided to revert back to '== null' checks.

Author: xeptore

src/filters.ts

@@ -4,7 +4,7 @@ import wordWrap from 'word-wrap';
 import { wordCase } from './utils';
 
 export function leadingBlankFilter(value: string, rule: RuleConfigTuple<undefined> | undefined): string {
-  if (rule === undefined) {
+  if (rule == null) {
     return value;
   }
 
@@ -26,7 +26,7 @@ export function leadingBlankFilter(value: string, rule: RuleConfigTuple<undefine
 }
 
 export function fullStopFilter(value: string, rule: QualifiedRules['subject-full-stop'] | undefined): string {
-  if (rule === undefined) {
+  if (rule == null) {
     return value;
   }
 

src/utils.ts

@@ -45,7 +45,7 @@ export function wordCase(value: string, rule: TargetCaseType): string {
 }
 
 export function valueFromRule<T>(rule: RuleConfigTuple<T> | undefined): undefined | T {
-  if (rule === undefined) {
+  if (rule == null) {
     return undefined;
   }
 

src/validators.ts

@@ -17,7 +17,7 @@ export function validate(
 ): string | true {
   const errorMessages: string[] = validators
     .map(v => {
-      if (v.rule === undefined) {
+      if (v.rule == null) {
         return true;
       }
 

src/when.ts

@@ -2,7 +2,7 @@ import type { RuleConfigTuple } from '@commitlint/types';
 import { RuleConfigSeverity } from '@commitlint/types';
 
 export function enumWhen(rule: RuleConfigTuple<string[]> | undefined): boolean {
-  if (rule === undefined) {
+  if (rule == null) {
     return true;
   }
 
@@ -22,7 +22,7 @@ export function enumWhen(rule: RuleConfigTuple<string[]> | undefined): boolean {
 }
 
 export function emptyWhen(rule: RuleConfigTuple<undefined> | undefined): boolean {
-  if (rule === undefined) {
+  if (rule == null) {
     return true;
   }