cgen: fix missing panic message for 'option not set' on debug (#18168)

vlib/v/gen/c/cgen.v

@@ -5958,7 +5958,7 @@ fn (mut g Gen) or_block(var_name string, or_block ast.OrExpr, return_type ast.Ty
 			err_msg := 'IError_name_table[${cvar_name}.err._typ]._method_msg(${cvar_name}.err._object)'
 			if g.pref.is_debug {
 				paline, pafile, pamod, pafn := g.panic_debug_info(or_block.pos)
-				g.writeln('panic_debug(${paline}, tos3("${pafile}"), tos3("${pamod}"), tos3("${pafn}"), ${err_msg} );')
+				g.writeln('panic_debug(${paline}, tos3("${pafile}"), tos3("${pamod}"), tos3("${pafn}"), ${err_msg}.len == 0 ? _SLIT("option not set ()") : ${err_msg});')
 			} else {
 				g.writeln('\tpanic_option_not_set( ${err_msg} );')
 			}

vlib/v/slow_tests/inout/option_panic.out

@@ -0,0 +1,5 @@
+================ V panic ================
+   module: main
+ function: main()
+  message: option not set ()
+     file: vlib/v/slow_tests/inout/option_panic.vv:7

vlib/v/slow_tests/inout/option_panic.vv

@@ -0,0 +1,8 @@
+fn t() ? {
+	a := ?string(none)
+	println(a?)
+}
+
+fn main() {
+	t()?
+}