Add null avoidance in rule require-optimization

[kamataryo]

Hello, thanks for the great ESLint rule.

This is an error report.
When the rule require-optimization encounters arrays with empty items as values in object (ex. { const obj = { prop: [,,,,,] } }), an error rises.

This PR provides null avoidance, simply.

[ljharb]

Thanks!

Sparse arrays are a horrific practice that massively slows down your code and should never be used, but it's definitely good that this plugin doesn't crash on them.