fix: gracefully handle empty hstore in pgdialect

[0rax]

In the current implementation of hstore in github.com/uptrace/bun/dialect/pgdialect, an empty hstore is encoded properly in the database as '' but can not be retrieved.

It is handled properly for a var md map[string]string = nil when NULL is inserted in database and an empty map is returned when querying back the record, but when using a map[string]string{} it will insert '' (which is expected) but will fail with the following error:

sql: Scan error on column index 8, name "metadata": bun: can't parse hstore: ""

This PR make sure an empty hstore is handled gracefully in the hstoreParser as well as adds test cases for this issues in pgdialect itself (for the parser AND the appender) as well as in the internal/dbtest package.

This fix is relatively crucial to us, as we tend to store user defined metadata in hstore and it may happen that a user removes all key value pairs from there. Currently, we have to check if the changes made to a metadata field makes it empty and force the field to be null, being able to handle empty hstore gracefully at the database driver level would make our life quite easier here as it seems like a legitimate use of that type.