Comparing two executions which do not have tags results in error

[mgencur]

Steps to reproduce:

1. Choose two points in Metric history report and add them to comparison
2. Click the "shopping basket" icon for comparing two test executions
3. See the following error
   /reports/compareExecutions.xhtml @79,91 value="#{compareExecutionsController.getTags(te)}": java.lang.StringIndexOutOfBoundsException: String index out of range: -1