Fix GitLab plugin with complex data
Sometimes receiving this error:
May 21 12:33:49 pmon1 alertad: INFO:alerta.plugins:Create GitLab issue for example / mweb1 / 123
May 21 12:33:49 pmon1 alertad: ERROR:flask.app:Error while running action plugin 'alerta-gitlab': Expecting value: line 1 column 1 (char 0)