Adding hstore entries to an uninitialized (NULL) column
In SQL, NULL (operator) (value) is generally NULL.

This is not unique to hstore, and is the norm for everything.

The empty string '' is different to NULL. '' || 'somestring' is 'somestring', wheras NULL || 'somestring' is NULL.

The same is true for hstore. Just like NULL + 1 is NULL.

If this is a problem for you, you should probably store empty hstore values instead of NULL and assign a NOT NULL constraint on the column.