Security/ValidatedSanitizedInput: improve InputNotValidated error message

The error message for `InputNotValidated` previously listed specific methods (isset() and empty()), but the list was incomplete. Rather than expanding it, this commit adopts a more generic message that won't require updates when new validation methods are added.

Fixes 2641

Co-authored-by: Juliette <[email protected]>

Author: rodrigoprimo

WordPress/Sniffs/Security/ValidatedSanitizedInputSniff.php

@@ -175,7 +175,7 @@ static function ( $var_name ) {
 
 		if ( false === $validated ) {
 			$this->phpcsFile->addError(
-				'Detected usage of a possibly undefined superglobal array index: %s. Use isset() or empty() to check the index exists before using it',
+				'Detected usage of a possibly undefined superglobal array index: %s. Check that the array index exists before using it.',
 				$stackPtr,
 				'InputNotValidated',
 				$error_data