Extension sets various environment variables to the string `"null"`

[timbertson]

I came across this as the root cause of https://github.com/microsoft/vscode-js-debug/issues/784#issuecomment-706633779

I assume this is happening when it's supposed to remove environment variables. I haven't dug into the code yet, my guesses are that either:

• the assignment code is assigning null to process.env[varname], but env internally forces environment variables to be strings so it's getting coerced
• it's incorrectly interpreting direnv's output for "unset variable" as "null" instead of null