Pipe character in a boosted field breaks indexing

After upgrading to 8.x-3.x, our indexing started breaking on particular nodes. After hours of debugging/experimenting, finally got to the bottom of the issue -- a pipe character in a field that was boosted, got added to the boost_term data incorrectly. In that field, the pipe is used to delimit the term from the boost weighting -- but the pipe was added by itself, with no term or weight, and sent to Solr this way.

Solr was throwing an exception, with this in the log:

> Exception writing document id o5zn56-content-entity:node/1412:en to the index; possible analysis error: empty String

... and indexing stopped at this node -- reindexing keeps failing in the same batch so at this point the index becomes stale.

Example in screenshot: