pcap SearchReader: Fix empty section headers about zed HOT 2 CLOSED

It sounds like the fix for this may also address a symptom we've run into where the output for a flow-not-found look like old-fashioned line noise. This was originally cited in pcap commit 5318ff6:

# pcap slice -p tcp -r all.pcap -x all.index 192.168.0.51:53001 80.239.174.122:80
?ò?no packets found

Technically this output was correct since it was the proper output for an empty pcap file, but if we can make this more user-friendly through better error handling, great!

from zed.

I've verified in pcap commit 4895043 that the symptom I was tracking has been addressed. No more "line noise". 😄

# pcap slice -p tcp -r zq-sample-data.pcap -x zq-sample-data.idx -w 0.pcap 192.168.0.51:53001 80.239.174.122:80 
item does not exist: no packets found

Thanks @mattnibs!

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