dialect/sql/schema: Atlas cleanSchema should handle non-existent tables (#3931)

sqlite fts5 virtual tables create additional tables that are automatically
deleted when the virtual table is deleted.

This will cause "no such table" errors when trying to drop them when creating
a new migration.

dialect/sql/schema/atlas.go

@@ -211,7 +211,7 @@ func (a *Atlas) cleanSchema(ctx context.Context, name string, err0 error) (err e
 	}
 	drop := make([]schema.Change, len(s.Tables))
 	for i, t := range s.Tables {
-		drop[i] = &schema.DropTable{T: t}
+		drop[i] = &schema.DropTable{T: t, Extra: []schema.Clause{&schema.IfExists{}}}
 	}
 	return a.atDriver.ApplyChanges(ctx, drop)
 }