fix: TypeError: Cannot convert undefined or null to object

[taufiqelrahman]

Issue to fix: https://github.com/Tencent/vConsole/issues/636

Description:

1. When page on load, the vconsole panel can be opened as well as the network panel.
2. After navigating around in my next.js app, the vconsole panel cannot be opened (after click) and there is error shows up: Uncaught (in promise) TypeError: Cannot convert undefined or null to object

Steps to Reproduce (if applicable):

1. Go forward and back in next.js app. （pages should perform requests）
2. click the vconsole floating panel.

Expected Behavior: The Network panel can be opened and track the network activities.

Actual Behavior: The vconsole panel and the Network panel cannot track the network activities.

Additional Information: Error message:

vconsole.min.js:10 Uncaught (in promise) TypeError: Cannot convert undefined or null to object
    at Function.entries (<anonymous>)
    at po (vconsole.min.js:10:230313)
    at Array.xo (vconsole.min.js:10:239253)
    at y (vconsole.min.js:10:123857)
    at Array.ue (vconsole.min.js:10:189716)
    at y (vconsole.min.js:10:123857)
    at Cn (vconsole.min.js:10:176259)
    at xt (vconsole.min.js:10:129051)
    at new e (vconsole.min.js:10:177517)
    at se (vconsole.min.js:10:190371)
    at bt (vconsole.min.js:10:127194)
    at Object.p (vconsole.min.js:10:189407)
    at Object.p (vconsole.min.js:10:193586)
    at lt (vconsole.min.js:10:126559)
    at st (vconsole.min.js:10:126239)

Proposed solution Handle when req.requestHeader is undefined