fix: use asserting to model if without else

[hargoniX]

This PR fixes the handling of non trivial if chains in sexpression model output. The key issue here
was the use of an if without else which is invalid syntax. Instead we use an asserting guard now.

Problematic part of the model before:

(if
 (and (= x_0 (l_BEq_elim__3_spec__18_mk $$anon_fun_0))
  (= x_1
   (l_List_elim__5_spec__16_cons
    $l___example_u945__8_elim__0_spec__15_0
    l_List_elim__5_spec__16_nil))
  (= x_2 l_List_elim__5_spec__16_nil) (= x_3 l_lt))
 l_Bool_elim__1_spec__13_false)

fixed part now:

(asserting l_Bool_elim__1_spec__13_false
  (and (= x_0 (l_BEq_elim__3_spec__18_mk $$anon_fun_0))
   (= x_1
    (l_List_elim__5_spec__16_cons
     $l___example_u945__8_elim__0_spec__15_0
     l_List_elim__5_spec__16_nil))
   (= x_2 l_List_elim__5_spec__16_nil) (= x_3 l_lt)))

Closes: #48