[Fixes #7391] MapLegend shows wrong legend instead od the selected (default) style

[marthamareal]

References: #7391

Consider a layer's default_style_name if the layer has a default style set.

Checklist

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[codecov]

Codecov Report

Merging #7394 (c44db0f) into master (5f45545) will increase coverage by 0.00%.
The diff coverage is 0.00%.

❗ Current head c44db0f differs from pull request most recent head e245837. Consider uploading reports for the commit e245837 to get more accurate results

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  Lines       41343    41345    +2     
  Branches     5433     5434    +1     
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+ Hits        24859    24861    +2     
- Misses      14991    14995    +4     
+ Partials     1493     1489    -4