Fix usuarios module: resolve PHP 8 warnings and fatal errors

- Fixed undefined array key warnings in usuarios-agregar.php
- Fixed undefined variable warnings (, , )
- Fixed null array offset warnings in db.class.php GetSingle()
- Added default parameter to ValidateMail() in util.class.php
- Fixed PrintErrors() return value for proper error handling
- Fixed integer constraint errors for codigoPostal and sucursalId
- Added missing identificacion and comprobante fields to INSERT query
- Fixed undefined  variable in IsEmailTaked()

classes/db.class.php

@@ -205,11 +205,9 @@ public function DatabaseConnect()
         } else {
             $row = mysqli_fetch_array($this->sqlResult);
         }
-		$rs = $row[0];
 
-		if(!$rs)
-			$rs = 0;
-			
+		$rs = ($row && isset($row[0])) ? $row[0] : 0;
+
     $this->CleanQuery();
 
     return $rs;