Shibboleth - Fix infinite loop for unexisting nuxeo user accounts

nuxeo@nuxeotraining:~/nuxeo-server-9.10-I20171205_2326-tomcat/bin$ nxc mp-list
Detected Tomcat server.
Nuxeo home:          /home/nuxeo/nuxeo-server-9.10-I20171205_2326-tomcat
Nuxeo configuration: /home/nuxeo/nuxeo-server-9.10-I20171205_2326-tomcat/bin/nuxeo.conf
Include template: /home/nuxeo/nuxeo-server-9.10-I20171205_2326-tomcat/templates/common-base
Include template: /home/nuxeo/nuxeo-server-9.10-I20171205_2326-tomcat/templates/common
Include template: /home/nuxeo/nuxeo-server-9.10-I20171205_2326-tomcat/templates/default
Include template: /home/nuxeo/nuxeo-server-9.10-I20171205_2326-tomcat/templates/mongodb
Include template: /home/nuxeo/nuxeo-server-9.10-I20171205_2326-tomcat/templates/shibboleth
Local packages:
 addon  downloaded	nuxeo-dam (id: nuxeo-dam-6.3.3-I20171206_0154)
 addon  downloaded	nuxeo-drive (id: nuxeo-drive-1.7.3-I20171206_0132)
 addon  downloaded	nuxeo-liveconnect (id: nuxeo-liveconnect-1.2.3-I20171206_0156)
 addon  downloaded	nuxeo-showcase-content (id: nuxeo-showcase-content-1.2.3-I20171206_0200)
 addon  downloaded	nuxeo-spreadsheet (id: nuxeo-spreadsheet-1.3.3-I20171206_0206)
 addon  downloaded	nuxeo-template-rendering (id: nuxeo-template-rendering-6.7.3-I20171206_0148)
 addon  downloaded	nuxeo-template-rendering-samples (id: nuxeo-template-rendering-samples-6.7.3-I20171206_0148)
 addon  downloaded	nuxeo-vision (id: nuxeo-vision-1.2.3-I20171206_0200)
 addon     started	nuxeo-web-ui (id: nuxeo-web-ui-2.1.4-I20171206_0203)
 addon     started	shibboleth-authentication (id: shibboleth-authentication-2.5.3-I20171206_0138)