This PR is to address the issue that builders will generate bad SQL statements if empty values and/or expressions are provided.

For instance, if we build SQL like following, we will get a SQL statement with syntax error.

// Intentionally let values be nil.
var values []int

sb := Select("c").From("t")
sb.Where(sb.In("c", ...Flatten(values)))

print(sb) // SELECT c FROM t WHERE c IN ()

In this PR, if there is no values or expressions provided in Cond.IN/Cond.NotIn/Cond.Some/Cond.All/Cond.Any/Cond.And/Cond.Or, the condition will be ignored completely. Thus, the case above will print SELECT c FROM t with this PR.

It's a better solution to address the same issue mentioned in PR #188. cc @ErikBooijMB