[red-knot] don't include Unknown in the type for a conditionally-defined import (#13563)

## Summary

Fixes the bug described in #13514 where an unbound public type defaulted
to the type or `Unknown`, whereas it should only be the type if unbound.

## Test Plan

Added a new test case

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/resources/mdtest/conditional/if_statement.md

@@ -28,6 +28,7 @@ else:
     y = 5
     s = y
 x = y
+
 reveal_type(x)  # revealed: Literal[3, 4, 5]
 reveal_type(r)  # revealed: Unbound | Literal[2]
 reveal_type(s)  # revealed: Unbound | Literal[5]