fix: Make the return value of accessToken nullable

[dshukertjr]

What kind of change does this PR introduce?

Bug fix.

Reading through the documentation, it looks like the return value of the accessToken should allow null to be returned, but the current type doesn't allow null to be returned. This PR updates the return type of the accessToken function to allow null to be returned.

What is the current behavior?

The code sample from the docs has type error, because null cannot be returned from accessToken function.

import { createClient } from '@supabase/supabase-js'

const supabase = createClient('https://<supabase-project>.supabase.co', 'SUPABASE_ANON_KEY', {
  accessToken: async () => {
    ;(await firebase.auth().currentUser?.getIdToken(/* forceRefresh */ false)) ?? null
  },
})

What is the new behavior?

null can be returned from the function if no users are found.

[coveralls]

Pull Request Test Coverage Report for Build 12290594479

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 67.633%

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Totals
Change from base Build 12280603551:	0.0%
Covered Lines:	99
Relevant Lines:	128

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💛 - Coveralls