PostgreSQLQueryBuilder returns invalid query with no columns to update

[hfaua]

Consider following code:

from pypika import PostgreSQLQuery as Query
from pypika import Table
t = Table("t")
q1 = Query.update(t).where(t.id == 1)
q2 = Query.update(t).where(t.id == 1).returning(t.star)

Those queries will result in following strings

>>> str(q1)
''
>>> str(q2)
' RETURNING "t".*'

The reason for such behaviour is that in PostgreSQLQueryBuilder.get_sql there is no check if super(PostgreSQLQueryBuilder, self).get_sql() returns an empty string.

Rationale:
You may be wondering if such a query is even valid and I would argue that it is and has some usages. My arguments are as follow:

1. When building an update query conditionally, it can happen that set() wouldn't be called because eg. a request wouldn't perform any update. See code below:

query = Query.update(t)
if param1 is not None:
    query = query.set(t.param1, param1)
if param2 is not None:
    query = query.set(t.param2, param2)

return execute(query.where(t.id = 1).returning(t.star))

In the above example te most elegant way IMO is to return an empty string for such query. Alternatives would be to either inspect private members QueryBuilder._updates (no-no) or checking if at least one parameter is not None (too much unnecessary code)

2. Behaviour of QueryBuilder.get_sql for an empty update query is defined in such a way that if no column is updated, then an empty string is returned. I think that for the sake of consistency, the implementation of PostgreSQLQueryBuilder should follow the same behaviour.

Proposal:
If you agree with me and I have your blessing, I can implement the fix. My fix would return empty string if super(PostgreSQLQueryBuilder, self).get_sql() returns an empty string. From what I can see, very similar situation also happens with PostgreSQLQueryBuilder.on_conflict, so this case will also be covered.

Three unit tests for the following queries will be added:

q1 = Query.update(t).where(t.id == 1).returning(t.star)
q2 = Query.into(t).on_conflict(t.a).do_nothing()
q3 = Query.into(t).returning(t.star)