Handle tables with missing PK

[fernandinand]

Currently, geodiff copy command fails if a given target tables does not have a PK implemented.
Despite this is not generally the best approach (not to have any PK), we need to handle this properly and provide some hints/output to user and not a error message.

[wonder-sk]

Note: this probably only affects postgres driver, which does not expect that there could no primary key and generates invalid CREATE TABLE command. Also worth checking diff & apply operations in postgres, whether they also fail on tables without primary key.