fix(HtmlParser): check if hashtag has text

Fixes a crash, where the text of an hashtag was empty, leading to an oob
string access.

mastodon/src/main/java/org/joinmastodon/android/ui/text/HtmlParser.java

@@ -140,7 +140,7 @@ public void head(@NonNull Node node, int depth){
 							String href=el.attr("href");
 							LinkSpan.Type linkType;
 							String text=el.text();
-							if(el.hasClass("hashtag") || text.startsWith("#")){
+							if(!TextUtils.isEmpty(text) && (el.hasClass("hashtag") || text.startsWith("#"))){
 								// MOSHIDON: we have slightly refactored this so that the hashtags properly work in akkoma
 								// TODO: upstream this
 								linkType=LinkSpan.Type.HASHTAG;