ResolvedNodes.ResolvedTableScan shouldn't require forSystemTimeExpr to be not null

Generated code validates that `forSystemTimeExpr` isn't null:

```java
protected void validate() {
super.validate();
Preconditions.checkArgument(this.table != null, "table must be set");
Preconditions.checkArgument(this.forSystemTimeExpr != null, "forSystemTimeExpr must be set");
Preconditions.checkArgument(this.alias != null, "alias must be set");
}
```

While `NULL` is a valid case if system time functionality isn't used.