NPE during mail generation after failed tests build

[ThishaniLucas]

Following NPE resulted when the mail was about to be created after the tests failed, in a k8s deployment. Mail is successfully sent when the tests pass. This issue happens only when the tests fail.

Exception in thread "main" java.lang.NullPointerException
	at org.wso2.testgrid.reporting.GraphDataProvider.processTestFailureSummary(GraphDataProvider.java:111)
	at org.wso2.testgrid.reporting.GraphDataProvider.getTestFailureSummary(GraphDataProvider.java:87)
	at org.wso2.testgrid.reporting.TestReportEngine.generateSummarizedEmailReport(TestReportEngine.java:836)
	at org.wso2.testgrid.core.command.GenerateEmailCommand.execute(GenerateEmailCommand.java:60)
	at org.wso2.testgrid.core.command.CommandHandler.execute(CommandHandler.java:130)
	at org.wso2.testgrid.core.Main.main(Main.java:47)