`Worksheet::getCell()` used to optionnaly return null if passed a
second argument. This second argument was removed entirely and the
method always returns a Cell (possibly creating it if needed).
This make the API more predictable and easier to do static analysis
with tools such as PHPStan.
If you relied on that second parameter, you should instead use the
`Worksheet::cellExists()` before calling `getCell()`.
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