Populate Reports tree children for a node only if reports exists.

The array that stores the reports (the children) for a particular Report
tree node was storing nils in the absence of the corresponding MiqReport
records, causing a "load error" in the Reports dynatree, which has
been fixed.

https://bugzilla.redhat.com/show_bug.cgi?id=1161744
Merge pull request #1060 from AparnaKarve/bz1161744_nil_entries_in_reports_dynatree

Populate Reports tree children for a node only if the MiqReport records exist.