Subtrace data not handled by any importer should be ignored.

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 
Let's drop UNKNOWN and set the default to DATA. Then we can remove the type = DATA from the majority of the importers.