resolves #20408

this pr tries to avoid the Using 'value' pointer is unsafe and deprecated. warning in the python bindings.

use a nullptr in createTrackbar(), and set the value later

Pull Request Readiness Checklist

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• I agree to contribute to the project under Apache 2 License.
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• There is reference to original bug report and related work
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  Patch to opencv_extra has the same branch name.
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