Conditional formatting - support for Conditional::CONDITION_CONTAINSBLANKS (#883)

Condition formating option was not supported. I unziped xlsx and found this formula to option blank field

src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php

@@ -519,6 +519,9 @@ class Worksheet extends WriterPart
                             // Formula
                             $objWriter->writeElement('formula', $formula);
                         }
+                    } elseif ($conditional->getConditionType() == Conditional::CONDITION_CONTAINSBLANKS) {
+                        // formula copied from ms xlsx xml source file
+                        $objWriter->writeElement('formula', 'LEN(TRIM(' . $cellCoordinate . '))=0');
                     }
 
                     $objWriter->endElement();