Replace SafetyEnumerable<T> with OfType<T> where applicable

[bahusoid]

Obsolete SafetyEnumerable<T> by replacing with OfType<T> extension.
Exception HQLQueryPlan where Cast is applied (as more appropriate behavior in this context)

[bahusoid]

IsFormula is not virtual and always false for Column

[fredericDelaporte]

There is only one place where it matters - HQLQueryPlan (as user can expect null from results).

This usage looks to me as a bug "hider", silently ignoring invalid result rows (not having the expected type). Shouldn't it be a Cast<T> here instead?

I wonder if the change is a good move, because in case some bug introduces a null value in those collections, the previous class would let it flow through and eventually cause trouble to consumer code, while with OfType it will be silently filtered out. But maybe that is more a trouble of missing null checks where we add things to these collections.

[bahusoid]

This usage looks to me as a bug "hider", silently ignoring invalid result rows (not having the expected type). Shouldn't it be a Cast here instead?

Yeah.. Indeed.. Only with special null handling for structs. Something like:
Select( obj => obj == null ? (default(T)) : (T)obj);
But I think such fix better to do in separate PR as possible breaking change.

But maybe that is more a trouble of missing null checks where we add things to these collections.

I would say yes to this. And regarding columns handling - null values didn't flow nicely to consumer code before. They stopped pretty quickly in AddColumns:

nhibernate-core/src/NHibernate/Mapping/Constraint.cs

Line 94 in 2f8f317

if (!col.IsFormula)

[bahusoid]

This usage looks to me as a bug "hider", silently ignoring invalid result rows (not having the expected type). Shouldn't it be a Cast here instead?

After another thought I made this change in this PR.