fix sighashes for generic ref (#20723)

the `hashType` issue in #12229 was never really fixed - it just stopped
being called in that context

compiler/sighashes.nim

@@ -185,6 +185,7 @@ proc hashType(c: var MD5Context, t: PType; flags: set[ConsiderFlag]) =
       hashType c, t[0], flags
   of tyRef, tyPtr, tyGenericBody, tyVar:
     c &= char(t.kind)
+    if t.sons.len > 0:
       c.hashType t.lastSon, flags
     if tfVarIsPtr in t.flags: c &= ".varisptr"
   of tyFromExpr: