[QA] CLC-4099 - Updated Canvas::SiteMembershipsMaintainer to filter out invalid SIS Section IDs upon initialization

https://jira.ets.berkeley.edu/jira/browse/CLC-4099

Updated Canvas::SiteMembershipsMaintainer to filter out invalid SIS Section IDs upon initialization

Master in #2835
I don't think this needs a warning. It's perfectly fine & expected for a Canvas section to lack a SIS section ID that doesn't connect to a campus section, and we've been quietly filtering them out from the start by letting sis_section_id_to_ccn_and_term do all the necessary checking for nil, blank, and not-the-integration-format:

https://github.com/ets-berkeley-edu/calcentral/blob/v41/app/models/canvas/batch_enrollments.rb#L11

The QA regression was due to our code not doing that anymore. That's a nasty bug for sure, but the fix just needs to be a one-liner somewhere.