MQE-455: trying to fill input with 0 does not generate properly

- changed if statement to is_null instead of empty, empty("0") = true

Author: KevinBKozan

dev/tests/verification/Resources/BasicFunctionalCest.txt

@@ -75,6 +75,7 @@ class BasicFunctionalCest
 		$I->dragAndDrop(".functionalTestSelector", ".functionalTestSelector2");
 		$I->executeJS("someJSFunction");
 		$I->fillField(".functionalTestSelector", "someInput");
+		$I->fillField(".functionalTestSelector", "0");
 		$grabAttributeVar = $I->grabAttributeFrom(".functionalTestSelector", "someInput");
 		$grabCookieVar = $I->grabCookie("grabCookieInput", ['domain' => 'www.google.com']);
 		$grabUrlVar = $I->grabFromCurrentUrl("/grabCurrentUrl");

dev/tests/verification/TestModule/Cest/basicFunctionalCest.xml

@@ -60,6 +60,7 @@
             <dragAndDrop selector1=".functionalTestSelector" selector2=".functionalTestSelector2" mergeKey="dragAndDropKey1" />
             <executeJS function="someJSFunction" mergeKey="executeJSKey1"/>
             <fillField selector=".functionalTestSelector" userInput="someInput" mergeKey="fillFieldKey1" />
+            <fillField selector=".functionalTestSelector" userInput="0" mergeKey="fillFieldKey2" />
             <grabAttributeFrom returnVariable="grabAttributeVar" selector=".functionalTestSelector" userInput="someInput" mergeKey="grabAttributeFromKey1" />
             <grabCookie returnVariable="grabCookieVar" userInput="grabCookieInput" parameterArray="['domain' => 'www.google.com']" mergeKey="grabCookieKey1" />
             <grabFromCurrentUrl returnVariable="grabUrlVar" url="/grabCurrentUrl" mergeKey="grabFromCurrentUrlKey1" />

src/Magento/FunctionalTestingFramework/Util/TestGenerator.php

@@ -1206,7 +1206,7 @@ private function addUniquenessFunctionCall($input)
      */
     private function wrapWithDoubleQuotes($input)
     {
-        if (empty($input)) {
+        if ($input == null) {
             return '';
         }
         //Only replace " with \" so that it doesn't break outer string.