return null if from ist not set in abstract stanza

src/main/java/eu/siacs/conversations/xmpp/stanzas/AbstractStanza.java

@@ -19,12 +19,17 @@ public class AbstractStanza extends Element {
 	}
 
 	public Jid getFrom() {
+		String from = getAttribute("from");
+		if (from == null) {
+			return null;
+		} else {
 			try {
-            return Jid.fromString(getAttribute("from"));
+				return Jid.fromString(from);
 			} catch (final InvalidJidException e) {
 				return null;
 			}
 		}
+	}
 
 	public String getId() {
 		return this.getAttribute("id");