fixes #25945; cannot map the empty seq type to a C type (#25954)

fixes #25945

When `@[]` appears inside a nested `if` expression that also contains
statements, the AST wraps it in `nkStmtListExpr` nodes. The empty
container's `tyEmpty` element type was never resolved to a concrete
type, causing the C codegen to ICE with "cannot map the empty seq type
to a C type".

Walk through nested statement-list/block expressions in
`fitNodePostMatch` to find the innermost value node and propagate the
formal type to empty containers.

tests/ccgbugs2/tcodegen.nim

@@ -75,3 +75,15 @@ block: # importc type inheritance
   doAssert(cast[cint](b) == 123)
   var c = foo(b)
   doAssert(cast[cint](c) == 123)
+
+block: # bug #25945
+  var stateRefund = 0
+  let authCode =
+    if true:
+      if false:
+        stateRefund += 0
+      @[]
+    else:
+      @([1.byte])
+
+discard (if true: (discard; @[]) else: @[0])