A null type node means a caller reached type resolution without a type
node. Every current caller passes a non-optional node or handles the
"no type" case itself (returning .void), so a null here is a caller bug;
.s64 silently fabricated an 8-byte int. Return the .unresolved sentinel
so it surfaces (trips the sizeOf/toLLVMType panic at codegen).
The only thing relying on the old behavior was a unit test asserting
null => .s64 -- i.e. a test pinning the silent default. Updated it to
pin .unresolved.
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