fix template error if no template string is defined
fix template error if no template string is defined

Kellel/alertmanager,sarathsp06/alertmanager,GoogleCloudPlatform/alertmanager,tbouvet/alertmanager,ericgerling/alertmanager,joehandzik/alertmanager,GoogleCloudPlatform/alertmanager,joehandzik/alertmanager,GoogleCloudPlatform/alertmanager,tyrken/alertmanager,ericgerling/alertmanager,Kellel/alertmanager,GoogleCloudPlatform/alertmanager,tbouvet/alertmanager,brancz/alertmanager,Aloomaio/alertmanager,Aloomaio/alertmanager,brunoqc/alertmanager,Kellel/alertmanager,brunoqc/alertmanager,brancz/alertmanager,tyrken/alertmanager,sarathsp06/alertmanager,prometheus/alertmanager,tinytub/alertmanager,brancz/alertmanager,tinytub/alertmanager,brunoqc/alertmanager,tyrken/alertmanager,joehandzik/alertmanager,prometheus/alertmanager,brancz/alertmanager,prometheus/alertmanager,Aloomaio/alertmanager,brunoqc/alertmanager,tyrken/alertmanager,prometheus/alertmanager,tinytub/alertmanager,sarathsp06/alertmanager,ericgerling/alertmanager,tbouvet/alertmanager