Add NULL support to Client DER library

client/src/der.vala

@@ -59,6 +59,8 @@ namespace StudySystemClient.Der {
 			return new Utf8String.from_content(content);
 		case Sequence.TYPE:
 			return new Sequence.from_content(content);
+		case Null.TYPE:
+			return new Null.from_content(content);
 		default:
 			throw new DecodeError.UNKNOWN_TYPE("Unsupported type: %02x",
 											   type);
@@ -287,4 +289,22 @@ namespace StudySystemClient.Der {
 			value = decode(bytes);
 		}
 	}
+
+	public class Null : Datum {
+		internal const uint8 TYPE = 0x05;
+
+		public Null() {
+			type = TYPE;
+			content = new uint8[] {};
+		}
+
+		internal Null.from_content(uint8[] bytes) throws DecodeError {
+			if (bytes.length != 0) {
+				throw new DecodeError.INVALID_CONTENT(
+					"Non-empty content for NULL");
+			}
+			type = TYPE;
+			content = bytes;
+		}
+	}
 }