isBefore() produces strange result when provided any one or both of the parameter as undefined.

**Describe the bug**
When we pass `undefined` as parameter in `dayjs().isBefore` then it sometimes it returns `true` and sometimes it returns `false`.
This breaks the code someone uses in condition;

*Steps to reproduce*
Step 1: Open chrome or any editor which has dayjs
Step 2: Log this piece of code 7-8 `dayjs(undefined).isBefore(undefined)` you might see different result.

**Screenshots**

A Clear & Better :

**Expected behavior**
Expected behaviour should be like if you pass both as undefined then it should return false every time or a single boolean (true or false any one them) OR it should be handled and thrown appropriate error.

**Information**
- Day.js Version [v1.11.8]
- OS: [macOS]
- Browser [chrome]
- Time zone: [e.g. GTT +5:30 IST (Asia/ Kolkata Time Zone)]