Various migration scripts define a normalize_raw method to do custom processing of post contents before storing it in the Post.raw and other fields.
They normally do not handle nil inputs, but it's a relatively common occurrence in data dumps.
Since this method is used from various points in the migration script, as it stands, the experience of using a migration script is that it will fail multiple times at different points, forcing you to fix the data or apply logic hacks every time then restarting.
This PR generalizes handling of nil input by returning a <missing> string.
Pros:
no more messy repeated crashes + restarts
consistency
Cons:
it might hide data issues
OTOH we can't print a warning on that method because it will flood the console since it's called from inside loops.
* FIX: zendesk import script: support nil inputs in normalize_raw
* FIX: return '<missing>' instead of empty string; do it for all methods
