feat: allow null or undefined to modifiers (#41)

This change makes sense for TypeScript users.

Author: ybiquitous

index.ts

@@ -1,5 +1,3 @@
-type Modifiers = { [key: string]: boolean; }
-
 const defaults = {
   elementDelimiter: '__',
   modifierDelimiter:  '--',
@@ -32,6 +30,10 @@ export function setup(options: {
   }
 }
 
+type Modifiers = {
+  [key: string]: boolean | null | undefined,
+}
+
 export default function bem(
   block: string,
   {

test.ts

@@ -119,6 +119,8 @@ testCases.forEach(({ description, tested, expectations }) => {
     it('returns block with modifier', () => {
       expect(b({ mod1: true })).toBe(expectations[1])
       expect(b({ mod1: true, mod2: false })).toBe(expectations[1])
+      expect(b({ mod1: true, mod2: null })).toBe(expectations[1])
+      expect(b({ mod1: true, mod2: undefined })).toBe(expectations[1])
     })
 
     it('returns block with multiple modifiers', () => {