Add type validation for $sheet_letters to prevent TypeError in PHP 8+

[Copilot]

The GoogleSheetsProcess constructor calls strtolower() on $sheet_letters without validating it's a string, causing TypeError when NULL/array is passed from misconfigured plugin settings.

Changes:

• Added is_string() check before string operations on $sheet_letters
• Defaults to empty string for invalid types (NULL, array, etc.)
• Empty string results in no columns being selected for processing

public function __construct($gsheet_returned_data, $sheet_letters, $process = 'ss') {
  // ... existing validation ...
  
  // Validate sheet_letters is a string before processing.
  // If not a string (e.g., NULL, array), default to empty string
  // which results in no columns being selected for processing.
  if (!is_string($sheet_letters)) {
    $sheet_letters = '';
  }

  // Sanitize sheet letters input.
  $sheet_letters = strtolower($sheet_letters);
  // ...
}

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