canonical_rrset in validate.py sometimes contain None

[emdee-is]

Maybe an edge case, but canonical_rrset in validate.py

    canonical_rrset = map(lambda rr: rr.canonical_l1(rrsig.original_ttl),
                          rrset)

sometimes contain None which throws an error on the next line in:

lambda x: binascii.hexlify(x.rdata)

I changed rrset to filter(bool, rrset) to squelch this - am I wrong?

Further down after for rr in rrset: I added if not rr: continue but that may be
overkill if the first change is made.

[metebalci]

Please tell me how I can re-generate the error, how you run it and attach the files etc. so I can debug.