Sometimes returns None for Valid HTTPS

# 🐛 Bug Report

pshtt sometimes returns `None` for the `Valid HTTPS` field. Originally reported by @climber-girl.

## To Reproduce

This error is difficult to reproduce. It happened with the following domains (and more) in the July 26-28 BOD scanning run:
* `icisairtestnode.epa.gov`
* `icisbatchcdxtest.epa.gov`
* `icisreportsxidev.epa.gov`
* `icisstagenode.epa.gov`
* `iciswsprod.epa.gov`
* `iciswsstage.epa.gov`
* `iciswstest.epa.gov`

## Expected behavior

The `Valid HTTPS` field should be `True` or `False`.

## Any helpful log output

Cross-referencing the CloudWatch logs and the pshtt source code, I can see that the error is due to an exception thrown by sslyze when analyzing certificates.