Issue
You are seeing discrepancies in how Heroku Connect syncs null and empty values from Salesforce.
Resolution
In Salesforce, values that aren't set have null values. This is different than an empty string, which is also a possible value.
There used to be a discrepancy in how Heroku Connect handled values that are defined as null in Salesforce, depending on whether it used the SOAP API or the Bulk API to retrieve the values.
However, that discrepancy was addressed in 2017. null values in Salesforce are now written as NULL values in Postgres.