RequestHeaders.Authorization parameter concatenation failed

[Allanbcruz]

Describe the bug

In situations that use Basic auth and do not pass the parameter "password" requestHeaders.Authorization is concatenated unduly ": undefined".

To Reproduce

Any request using Basic Authentication without passing the parameter "password" as an example below.

var axios = require("axios")
const instance = axios.create();
instance.request({
    method: 'POST',
    baseURL: ".../XPTO",
    auth: { username: "janedoe" }}
    ).then(
        // do something
    );

Expected behavior

To maintain compatibility with previous versions, the proper behavior would be to test if the parameter "password" was passed, otherwise concatenate empty.

Environment

• Axios Version [e.g. 0.20.0]
• Adapter [e.g. XHR/HTTP]
• Browser [e.g. Chrome, Safari]
• Browser Version [e.g. 22]
• Node.js Version [e.g. 13.0.1]
• OS: [e.g. iOS 12.1.0, OSX 10.13.4]
• Additional Library Versions [e.g. React 16.7, React Native 0.58.0]

Additional context/Screenshots

A problem arising from the lack of compatibility with previous versions is to force all current projects that do not pass a "password" parameter to have to include this parameter with an empty value.