Prevent Gitlab URL from being null (jenkinsci#1187)

Author: mkemmerz

src/main/java/com/dabsquared/gitlabjenkins/connection/GitLabConnection.java

@@ -90,7 +90,7 @@ public GitLabConnection(String name, String url, String apiTokenId, String clien
     @Restricted(NoExternalUse.class)
     public GitLabConnection(String name, String url, String apiTokenId, GitLabClientBuilder clientBuilder, boolean ignoreCertificateErrors, Integer connectionTimeout, Integer readTimeout) {
         this.name = name;
-        this.url = url;
+        this.url = url == null ? "" : url;
         this.apiTokenId = apiTokenId;
         this.clientBuilder = clientBuilder;
         this.ignoreCertificateErrors = ignoreCertificateErrors;