Incorrect SQL query when there are no updates in the PostgreSQL table

If there are no changes in the PostgreSQL table, geodiff generates wrong query to fetch updated columns:
```
UPDATE "schema"."table" SET  WHERE "id" = 2
```
As a result it fails to create changeset with the error
```
Error: apply changeset failed!
GEODIFF: Error: postgres cmd error: ERROR:  syntax error at or near "WHERE"
LINE 1: ... "schema"."table" SET  WHERE "id"...
```