docs: fix documented return value for getFocusedWebContents

[kevinushey]

Description of Change

Fixes a small documentation issue -- webContents.getFocusedWebContents() can return null, so that value should be part of the return type signature. Thanks to @erickzhao for the assist and pointers.

See also: electron/typescript-definitions#202

Checklist

• relevant documentation is changed or added

Release Notes

Notes: Fixed the documented return value declared by webContents.getFocusedWebContents() to include null, matching the documented usage of the function.

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[jkleinsc]

Merging as CI failure unrelated to PR change.

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[release-clerk]

Release Notes Persisted

Fixed the documented return value declared by webContents.getFocusedWebContents() to include null, matching the documented usage of the function.