Fix headerToToken value check from null to undefined

`headerToToken` returns `Promise<string | undefined>`. However, the existing
check assessed `headerToToken().then(value => value === null ? something
: somethingElse)`.
This should never be true.
Instead, this check is replaced with `!value`.

src/background/index.ts

@@ -295,7 +295,7 @@ export const handleHeadersReceived =
         }
         const redirectPromise = w3HeaderValue
             .then(async (value): Promise<void | chrome.webRequest.BlockingResponse> => {
-                if (value === null) {
+                if (!value) {
                     delete TOKENS[details.url];
                     return;
                 }