onConflictIgnore() with an empty set of fields generates invalid SQL

[aboisvert]

Databases like Postgres support ON CONFLICT IGNORE without specifying fields to allow ignoring the INSERT/UPDATE statement on any duplicate key violation.

Currently scalasql seems to generate invalid SQL when the fields clause is empty,

org.postgresql.util.PSQLException: ERROR: syntax error at or near ")"
  Position: 117
	at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2733)
	at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:2420)
	at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:372)
	at org.postgresql.jdbc.PgStatement.executeInternal(PgStatement.java:517)
	at org.postgresql.jdbc.PgStatement.execute(PgStatement.java:434)
	at org.postgresql.jdbc.PgPreparedStatement.executeWithFlags(PgPreparedStatement.java:194)
	at org.postgresql.jdbc.PgPreparedStatement.executeQuery(PgPreparedStatement.java:137)
	at com.zaxxer.hikari.pool.ProxyPreparedStatement.executeQuery(ProxyPreparedStatement.java:52)
	at com.zaxxer.hikari.pool.HikariProxyPreparedStatement.executeQuery(HikariProxyPreparedStatement.java)
	at scalasql.core.DbApi$Impl$$anon$1.generate$$anonfun$2(DbApi.scala:473)
	at scalasql.core.DbApi$Impl.configureRunCloseStatement(DbApi.scala:563)
	at scalasql.core.DbApi$Impl$$anon$1.generate(DbApi.scala:472)
	at geny.Generator.foreach(Generator.scala:51)
	at geny.Generator.foreach$(Generator.scala:33)
	at scalasql.core.DbApi$Impl$$anon$1.foreach(DbApi.scala:460)
	at geny.Generator.toBuffer(Generator.scala:132)
	at geny.Generator.toBuffer$(Generator.scala:33)
	at scalasql.core.DbApi$Impl$$anon$1.toBuffer(DbApi.scala:460)
	at geny.Generator.toVector(Generator.scala:140)
	at geny.Generator.toVector$(Generator.scala:33)
	at scalasql.core.DbApi$Impl$$anon$1.toVector(DbApi.scala:460)
	at scalasql.core.DbApi$Impl.run(DbApi.scala:276)

I intend to submit a PR when I have time, just reporting for awareness at this time