Ignore empty values and expressions to prevent syntax error

[huandu]

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

[coveralls]

coverage: 96.732% (-0.1%) from 96.879%
when pulling fc56bc8 on feature/prevent-syntax-error-with-zero-values
into b0b0b02 on master.

[ErikBooijMB]

Sorry, totally missed this PR in in my notification. Really appreciate your effort to jump on this and fix it in a way that's cleaner than my approach was.

Not 100% sure that silently ignoring these conditions with no values is right approach. For NOT IN I think that's fine, and yields the expected result, but for IN/ANY/SOME/ALL, this may do the opposite of what a user expected.

[ErikBooijMB]

I'm wondering if this is what users will intuitively expect. I wouldn't. If I have an "allowlist" of values that I'm passing here, and it's empty for some reason, that query should yield no results, not silently ignore the filter and thus actually widen the result set.