fix: Type error on code exchange when no item in storage (#825)

## What kind of change does this PR introduce?

Currently when code exchange for pkce flow is triggered on device that
didn't start the auth flow (e.g. signed up via magic link on laptop and
clicked on the link on phone), `TypeError: Cannot read properties of
null (reading 'split')` error is thrown. This PR fixes that.

Fixes the error pointed out here
#813 (comment)

src/GoTrueClient.ts

@@ -515,9 +515,8 @@ export default class GoTrueClient {
       }
     | { data: { session: null; user: null; redirectType: null }; error: AuthError }
   > {
-    const [codeVerifier, redirectType] = (
-      (await getItemAsync(this.storage, `${this.storageKey}-code-verifier`)) as string
-    ).split('/')
+    const storageItem = await getItemAsync(this.storage, `${this.storageKey}-code-verifier`)
+    const [codeVerifier, redirectType] = ((storageItem ?? '') as string).split('/')
     const { data, error } = await _request(
       this.fetch,
       'POST',