Bugfix issue 60

I assume this fixes #72, #60 and #70

I already deployed a custom build with this fix on our production server.

NPE which is fixed by the second bugfix in commit 961a045
```
java.lang.NullPointerException
        at org.jenkinsci.plugins.gitlab.GitlabMergeRequestWrapper.check(GitlabMergeRequestWrapper.java:93)
        at org.jenkinsci.plugins.gitlab.GitlabRepository.check(GitlabRepository.java:83)
        at org.jenkinsci.plugins.gitlab.GitlabRepository.check(GitlabRepository.java:65)
        at org.jenkinsci.plugins.gitlab.GitlabMergeRequestBuilder.run(GitlabMergeRequestBuilder.java:27)
        at org.jenkinsci.plugins.gitlab.GitlabBuildTrigger.run(GitlabBuildTrigger.java:100)
        at hudson.triggers.Trigger.checkTriggers(Trigger.java:266)
        at hudson.triggers.Trigger$Cron.doRun(Trigger.java:214)
        at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:54)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
        at java.util.concurrent.FutureTask$Sync.innerRunAndReset(FutureTask.java:317)
        at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:150)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$101(ScheduledThreadPoolExecutor.java:98)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.runPeriodic(ScheduledThreadPoolExecutor.java:181)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:205)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
        at java.lang.Thread.run(Thread.java:619)
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