fix segfault with gensym node instantiation (#24050)

fixes #24048

Generic lambdas get instantiated via `replaceTypesInBody` which calls
`replaceTypeVarsN` on the body of the lambda. This body can contain sym
nodes of gensym symbols generated by macros, which have `nil` type. But
a piece of code in `replaceTypeVarsN` checks whether the type of a
symbol is equal to `void` without checking if it's `nil` first, which
causes a segfault. Now it also checks that the type of the symbol isn't
`nil` for it to be `void`.

tests/generics/tgensyminst.nim

@@ -0,0 +1,29 @@
+# issue #24048
+
+import macros
+
+proc map(fn: proc(val: int): void) = fn(1)
+
+# This works fine, and is the exact same function call as what's
+# generated by the macro `aBug`.
+map proc(val: auto): void =
+  let variable = 123
+
+macro aBug() =
+  # 1. let sym = ident("variable")
+  let sym = genSym(nskLet, "variable")
+  let letStmt = newLetStmt(sym, newLit(123))
+
+  let lambda = newProc(
+    params = @[
+      ident("void"),
+      newIdentDefs(ident("val"), ident("auto")),
+      # 2. newIdentDefs(ident("val"), ident("int")),
+    ],
+    body = newStmtList(letStmt),
+    procType = nnkLambda
+  )
+
+  result = newCall(bindSym("map"), lambda)
+
+aBug()