don't emit .space with 0 bytes

ir/be/begnuas.c

@@ -1602,8 +1602,11 @@ static void emit_global(be_gas_decl_env_t *env, const ir_entity *entity)
 	}
 
 	if (entity_is_null(entity)) {
-		be_emit_irprintf("\t.space %u, 0\n", get_type_size_bytes(type));
-		be_emit_write_line();
+		unsigned size = get_type_size_bytes(type);
+		if (size > 0) {
+			be_emit_irprintf("\t.space %u, 0\n", get_type_size_bytes(type));
+			be_emit_write_line();
+		}
 	} else if (entity_has_compound_ent_values(entity)) {
 		emit_compound_graph_init(env, entity);
 	} else {