Fix bug for tables without geom or with no potential overviews
Fix bug for tables without geom or with no potential overviews

If the table doesn't have geometries but the createoverviews function is
called, the current geometry type checks won't work because "null" will
not give a boolean value in the type comparisons.

Also, if the createoverviews function is called over a simple table with
would not require overviews according to the strategies it is handled
correctly.

CartoDB/cartodb-postgresql,CartoDB/cartodb-postgresql