Don't set :initform nil if the type does not allow it

[ruricolist]

As of SBCL 2.0.9, "the compiler signals a warning at compile-time when an initform of T, NIL or 0 does not match a STANDARD-CLASS slot's declared type".

This adds a compile-time check that prevents :initform nil from being inserted in the expansion if null is not a subtype of the slot's declared type.

[EuAndreh]

"Paul M. Rodriguez" <notifications@github.com> writes: Thanks for the patch! 0300f17..a4d3226 master -> master