Fix handling of case range statement in special case

In the case where a gcc case range switch statement has
a case that reduces to an empty expression, both gcc and
clang reluctantly accept it, producing warnings. CBMC up
until now was asserting against that case, and this
divergence in behaviour was unwanted.

Author: NlightNFotis

regression/ansi-c/goto_convert_switch_range_empty/test.desc

@@ -1,4 +1,4 @@
-KNOWNBUG
+CORE
 main.c
 
 ^EXIT=0$

src/goto-programs/goto_convert.cpp

@@ -1249,7 +1249,8 @@ void goto_convertt::convert_switch(
   {
     const caset &case_ops=case_pair.second;
 
-    assert(!case_ops.empty());
+    if (case_ops.empty())
+      continue;
 
     exprt guard_expr=case_guard(argument, case_ops);