cgen: fix sumtype with none type (fix #12101) (#12102)

vlib/v/gen/c/cgen.v

@@ -6038,6 +6038,12 @@ fn (mut g Gen) write_types(types []ast.TypeSymbol) {
 		if typ.name.starts_with('C.') {
 			continue
 		}
+		if typ.kind == .none_ {
+			g.type_definitions.writeln('struct none {')
+			g.type_definitions.writeln('\tEMPTY_STRUCT_DECLARATION;')
+			g.type_definitions.writeln('};')
+			g.typedefs2.writeln('typedef struct none none;')
+		}
 		// sym := g.table.get_type_symbol(typ)
 		mut name := typ.cname
 		match mut typ.info {

vlib/v/tests/sum_type_with_none_type_test.v

@@ -0,0 +1,5 @@
+type MapValue = int | none
+
+fn test_sum_type_with_none_type() {
+	assert true
+}