Fix errors when some entries were missing when retrieving account information (#757)

* Handle null avatar

* Handle empty account

mastodon/src/main/java/org/joinmastodon/android/api/session/AccountSession.java

@@ -313,4 +313,10 @@ public class AccountSession{
 				.authority(getInstance().map(i -> i.normalizedUri).orElse(domain))
 				.build();
 	}
+
+	public String getDefaultAvatarUrl() {
+		return getInstance()
+				.map(instance->"https://"+domain+(instance.isAkkoma() ? "/images/avi.png" : "/avatars/original/missing.png"))
+				.orElse("");
+	}
 }