fix(api): falsy values in preConfirm (#1403)

* Changed check on preConfirmValue

Currently the check on preConfirmValue is done on a falsy value. This include also 0,
which could be a valid return value.

Replaced it with a check for udnefined value, which is the only case
where we may want to return something else

* Added test for preConfirm returning undefined

* Changed test for preConfirm return value

Testing the code that causes the issue this time.

src/instanceMethods/_main.js

@@ -101,7 +101,7 @@ export function _main (userParams) {
             if (dom.isVisible(domCache.validationMessage) || preConfirmValue === false) {
               this.hideLoading()
             } else {
-              succeedWith(preConfirmValue || value)
+              succeedWith(typeof (preConfirmValue) === 'undefined' ? value : preConfirmValue)
             }
           }
         )

test/qunit/tests.js

@@ -766,3 +766,18 @@ QUnit.test('Custom content', (assert) => {
     done()
   })
 })
+
+QUnit.test('preConfirm returns 0', (assert) => {
+  const done = assert.async()
+  SwalWithoutAnimation.fire({
+    onOpen: () => {
+      Swal.clickConfirm()
+    },
+    preConfirm: () => {
+      return 0
+    }
+  }).then(result => {
+    assert.equal(result.value, 0)
+    done()
+  })
+})