Setting environment variable with empty name on Windows causes assertion failure

* **Version**: >=12.11.0
* **Platform**: Windows
* **Subsystem**: src

Setting an environment variable with an empty name on Windows causes an assertion failure:

```
> process.env[''] = ''
C:\Windows\System32\cmd.exe - node.exe[11140]: c:\ws\src\util.h:336: Assertion `(index) < (length())' failed.
```