Fix Panel logic for selecting priority of title/subtitle styling

[noprobelm]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

This is a proposed fix for the issue described in #2745.

• The existing behavior of the Panel.__rich_console__ method is to check if title_text and subtitle_text are present, and if so, apply the Panel's border style indicated by border_style.
• My proposed change is to extend the existing conditional statements to refrain from setting the styles for title_text and subtitle_text if they were already defined in the Panel instance when the console protocol was initiated.