Correct handling of Void response_bytes in OCSP

[geitda]

response_bytes is optional, thus Void when omitted, which happens with some statuses (like 'unauthorized'). Make all properties return correct values in this case: None for everything except critical_extensions which is the empty set.
I wrote it with a guard that causes the indent to increase - could be rewritten as

if not self['response_bytes']
    self._processed_extensions = True
    return

if early return and repeating self._processed_extensions = True is deemed less bothersome than the increased indent.

The simple ternary operator fix for basic_ocsp_response and response_data may not be enough for every case as there are several more layers of unwrapping that need to be error-free, but looking at #246 and #262 this should fix the TypeError although user code will (still) have to check for None but the docstrings always indicated that possibility.

The test code I didn't write might include this - it's what a real 'unauthorized' response looks like (I got it from Let's Encrypt when querying an expired cert)

resp = ocsp.OCSPResponse.load(b'0\x03\n\x01\x06') # only 5 bytes

Yes, that short: a sequence with an enumerated '6' value, and the response_bytes is completely omitted.