[pickers] Fix date format resolver on `DateTimeRangePickers`

[arthurbalduini]

For DateTimeRangePickers the ["day"] case returns an incorrect format, as reported in #14334

[mui-bot]

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[arthurbalduini]

I've adjusted the approach to one that looks more correct to me. I'm not sure why we would have differentiated the behavior based on the presence of value. 🤔

Let me know what you think about the solution. 😉

I assumed that veryfing if the value is an array (date range) would be more clear than adding a new parameter to the function to be used only in DateTimePickers, since the view verification was done in resolveDateFormat(). Let's go with your solution