Allow build to be triggered when pushing to an Open MR

[daddykotex]

The function generateMergeRequestBuild return when it should not be if the state
equals to "update". I removed the check to allow the build to be triggered.

I also used StringUtils.isEmpty to perform a check in GitLabWebHook to avoid a NPE.

[jenkinsadmin]

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[jtheoof]

Looks good, seems to be fixing #90 Works fine with our dev environment.

[coder-hugo]

Open MRs will be triggered already by pushing to the corresponding branch if the option "Rebuild open Merge Requests" is set at least to "On push to source branch" (Tested with GitLab 7.14.3 and GitLab 8.4.3 using the code of PR #207 which should mainly contain code cleanups and no bug fixes)
If we use the update Merge Request Event to trigger builds for a open MR (which IMHO is the cleaner way as we don't need to figure out the MR for an incoming Push Event) we should remove the code that triggers a new MR build in case of a Push Event.

[daddykotex]

Looks like @coder-hugo did all the work in #207 , thank you for that!