fix: falsy arg default

src/parse.ts

@@ -217,7 +217,7 @@ export class Parser<T extends ParserInput, TFlags extends OutputFlags<T['flags']
           args[i] = token.input
         }
       } else {
-        if (arg.default) {
+        if ('default' in arg) {
           if (typeof arg.default === 'function') {
             args[i] = arg.default()
           } else {

test/parse.test.ts

@@ -324,6 +324,13 @@ See more help with --help`)
       expect(out.flags).to.deep.include({foo: 'bar'})
     })
 
+    it('accepts falsy', () => {
+      const out = parse([], {
+        args: [{name: 'baz', default: false}],
+      })
+      expect(out.args).to.deep.include({baz: false})
+    })
+
     it('default as function', () => {
       const out = parse([], {
         args: [{name: 'baz', default: () => 'BAZ'}],