Non-json eventData to be handled by TraceModelEmptyImporter

When importing zip, gzip traces, the archive might contain eventData
which is neither json nor empty string. Trace-viewer doesn't know
how to handle that eventData and it throws no importer error.

Any eventData which is not json, should be treated as empty string
and should be handled by TraceModelEmptyImporter itself.

BUG= #613 
This isn't correct. Trace files can start with a [ or a { or, they can even start with HTML and then have the trace data show up later. We parse the trace data out of the HTML files if needed.

I think the correct solution to this is to try the importer and catch the error, and then handle appropriately. If there is no importer, just log an error and check the next file. If there are no valid files, then show the warning dialog.