wechat_qrcode: fix ineffective decoding retrial after inverting image

[Doekin]

This PR fixes a logic error in the QRCodeReader::decode function
when handling inverted images. Previously, the function would return
an empty result list even when successful decoding occurred on an
inverted image. The change ensure that:

1. If decoding fails on the original image, we attempt decoding on an inverted image.
2. If decoding succeeds on the inverted image, we now correctly return those results.
3. We maintain the behavior of returning an empty list if both original and inverted image decoding fail.

Pull Request Readiness Checklist

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• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake