Don't draw an e-r graph if no data

If there are no data for a bug query, just don't try to draw the graph.
Apparently the flot lib had a recent change which breaks drawing graphs
with no data.  This seems like a sensible idea anyway.

Instead the string "No matches" is substituted for the graph.

Change-Id: I3733974a9ec180fef2ca74c5e799155cadd07b7d
Merge "Don't draw an e-r graph if no data"