Add empty cell condition for conditional formatting (#128)

src/PhpSpreadsheet/Style/Conditional.php

@@ -31,6 +31,7 @@ class Conditional implements \PhpOffice\PhpSpreadsheet\IComparable
     const CONDITION_CELLIS = 'cellIs';
     const CONDITION_CONTAINSTEXT = 'containsText';
     const CONDITION_EXPRESSION = 'expression';
+    const CONDITION_CONTAINSBLANKS = 'containsBlanks';
 
     /* Operator types */
     const OPERATOR_NONE = '';