Acceps 'null' as build status

To prevent exceptions during parsing. This solves f.e:
```java
Error during scheduled task
java.lang.RuntimeException: java.lang.IllegalStateException: 'null' is not a valid build status
	at play.db.jpa.JPA.withTransaction(JPA.java:108) ~[com.typesafe.play.play-java-jpa_2.10-2.2.1.jar:2.2.1]
	at Global$1.run(Global.java:29) ~[analysisdashboard.analysisdashboard-1.0-SNAPSHOT.jar:na]
	at akka.actor.LightArrayRevolverScheduler$$anon$3$$anon$2.run(Scheduler.scala:241) [com.typesafe.akka.akka-actor_2.10-2.2.0.jar:2.2.0]
	at akka.dispatch.TaskInvocation.run(AbstractDispatcher.scala:42) [com.typesafe.akka.akka-actor_2.10-2.2.0.jar:2.2.0]
	at akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:386) [com.typesafe.akka.akka-actor_2.10-2.2.0.jar:2.2.0]
	at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260) [org.scala-lang.scala-library-2.10.2.jar:na]
	at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339) [org.scala-lang.scala-library-2.10.2.jar:na]
	at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979) [org.scala-lang.scala-library-2.10.2.jar:na]
	at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107) [org.scala-lang.scala-library-2.10.2.jar:na]
Caused by: java.lang.IllegalStateException: 'null' is not a valid build status
	at jsonhandling.BuildStatus.fromString(BuildStatus.java:31) ~[analysisdashboard.analysisdashboard-1.0-SNAPSHOT.jar:1.0-SNAPSHOT]
	at jsonhandling.RunParser.getStatus(RunParser.java:15) ~[analysisdashboard.analysisdashboard-1.0-SNAPSHOT.jar:1.0-SNAPSHOT]
	at analysis.analyzers.RunAnalyzer.analyze(RunAnalyzer.java:40) ~[analysisdashboard.analysisdashboard-1.0-SNAPSHOT.jar:1.0-SNAPSHOT]
	at analysis.analyzers.JobAnalyzer.analyzeDetails(JobAnalyzer.java:96) ~[analysisdashboard.analysisdashboard-1.0-SNAPSHOT.jar:1.0-SNAPSHOT]
	at analysis.analyzers.JobAnalyzer.analyze(JobAnalyzer.java:55) ~[analysisdashboard.analysisdashboard-1.0-SNAPSHOT.jar:1.0-SNAPSHOT]
	at analysis.analyzers.ViewAnalyzer.analyze(ViewAnalyzer.java:58) ~[analysisdashboard.analysisdashboard-1.0-SNAPSHOT.jar:1.0-SNAPSHOT]
	at analysis.analyzers.JenkinsServerAnalyzer.analyzeViews(JenkinsServerAnalyzer.java:71) ~[analysisdashboard.analysisdashboard-1.0-SNAPSHOT.jar:1.0-SNAPSHOT]
	at analysis.executor.AnalysisExecutor.analyzeServer(AnalysisExecutor.java:102) ~[analysisdashboard.analysisdashboard-1.0-SNAPSHOT.jar:1.0-SNAPSHOT]
	at analysis.executor.AnalysisExecutor.executeAnalysis(AnalysisExecutor.java:71) ~[analysisdashboard.analysisdashboard-1.0-SNAPSHOT.jar:1.0-SNAPSHOT]
	at Global$1$1.invoke(Global.java:34) ~[analysisdashboard.analysisdashboard-1.0-SNAPSHOT.jar:na]
	at play.db.jpa.JPA$1.apply(JPA.java:103) ~[com.typesafe.play.play-java-jpa_2.10-2.2.1.jar:2.2.1]
	at play.db.jpa.JPA$1.apply(JPA.java:101) ~[com.typesafe.play.play-java-jpa_2.10-2.2.1.jar:2.2.1]
	at play.db.jpa.JPA.withTransaction(JPA.java:132) ~[com.typesafe.play.play-java-jpa_2.10-2.2.1.jar:2.2.1]
	at play.db.jpa.JPA.withTransaction(JPA.java:101) ~[com.typesafe.play.play-java-jpa_2.10-2.2.1.jar:2.2.1]
	... 8 common frames omitted
```