Most bridges don't have a transport type

Looking at the recent bridge pool assignment stats, the vast majority of bridges aren't reporting a transport type.

$ grep "obfs4" Downloads/2021-02-07-23-30-26 | wc -l
181
$ wc -l Downloads/2021-02-07-23-30-26 
1547 Downloads/2021-02-07-23-30-26

I noticed this when I used the advance options to try requesting an obfs4 bridge with IPv6 and got a message saying there were no bridges in that ring. If I set transport type to none I get a vanilla bridge but no obfs4 bridges. I asked @phw about this the other day and he said that almost all of our bridges should be obfs4. If that's the case, there seems to be a bug somewhere.