fix (native): no password prompt on db update

[betodealmeida]

When doing an asset sync, if a database YAML file has a masked password (eg, postgresql://user:XXXXXXXXXX@host:5432/dbname) and the database already exists, don't prompt users for the password. We can simply pass the masked password and Superset will do the right thing.

Also fix a dataset bug I found when exporting the examples database. For some reason the params attribute in the dataset is a string:

table_name: Video Game Sales
schema: public
sql: ''
params: '{"remote_id": 64, "database_name": "examples", "import_time": 1606677834}'
...

Even though it should be an object:

https://github.com/apache/superset/blob/d3f930a5575f03bc5f4f468b8bf37e834bf2aa4d/superset/datasets/commands/export.py#L61