fix writing files without residues

src/dssp.cpp

@@ -318,6 +318,13 @@ void annotateDSSP(mmcif::Structure& structure, const mmcif::DSSP& dssp, std::ost
 {
 	auto& db = structure.getFile().data();
 
+	if (dssp.empty())
+	{
+		if (cif::VERBOSE)
+			std::cout << "No secondary structure information found" << std::endl;
+	}
+	else
+	{
 		// replace all struct_conf and struct_conf_type records
 		auto& structConfType = db["struct_conf_type"];
 		structConfType.clear();
@@ -429,9 +436,9 @@ void annotateDSSP(mmcif::Structure& structure, const mmcif::DSSP& dssp, std::ost
 				foundTypes[id] = 1;
 			}
 		}
+	}
 
 	db.add_software("dssp " VERSION, "other", get_version_nr(), get_version_date());
-
 	db.write(os);
 
 // 	cif::File df;