Fix: Default to UTF-8 for empty blog_charset values

[Sukhendu2002]

Trac ticket: https://core.trac.wordpress.org/ticket/25693

Changes Made

• Added a new function _wp_ensure_blog_charset in wp-includes/option.php that returns 'UTF-8' when the charset is empty
• Registered this function as a filter on 'option_blog_charset' with priority 9 in default-filters.php to ensure it runs before other charset-handling functions
• Modified is_utf8_charset and _canonical_charset functions in functions.php to properly handle empty values
• Added comprehensive unit tests to verify the changes

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