fix: fallback to hashtag name

Fixes an issue, where Hashtag timeline where created with an empty
hashtag.

mastodon/src/main/java/org/joinmastodon/android/fragments/EditTimelinesFragment.java

@@ -396,7 +396,7 @@ protected void makeTimelineEditor(@Nullable TimelineDefinition item, Consumer<Ti
 					tl.setTitle(name);
 					if(item == null || item.getType()==TimelineDefinition.TimelineType.HASHTAG){
 						tl.setTagOptions(
-								mainHashtag,
+								TextUtils.isEmpty(mainHashtag) ? name : mainHashtag,
 								tagsAny.getChipValues(),
 								tagsAll.getChipValues(),
 								tagsNone.getChipValues(),