Fix empty values sent in dynamic form (#3886)
Fix empty values sent in dynamic form (#3886)

* remove legacy session identifier support

* remove redundant test

* redirect to login to support any invalid session identifiers

* be more specific with caught errors

* reject empty values in DynamicForm

* don't submit form values if they are empty (unless they are
intentionally set to empty string)

* set empty values to null to clear out data source option in the model

* check explicitly for null

alexanderlz/redash,denisov-vlad/redash,denisov-vlad/redash,denisov-vlad/redash,alexanderlz/redash,getredash/redash,alexanderlz/redash,getredash/redash,getredash/redash,denisov-vlad/redash,getredash/redash,getredash/redash,denisov-vlad/redash,alexanderlz/redash