Add StringUtils.NullIfEmpty & update existing code to use it

lib/TweetLib.Core/Features/Twitter/TwitterUrls.cs

@@ -1,6 +1,7 @@
 using System;
 using System.IO;
 using System.Text.RegularExpressions;
+using TweetLib.Core.Utils;
 
 namespace TweetLib.Core.Features.Twitter{
     public static class TwitterUrls{
@@ -22,8 +23,7 @@ public static bool IsTwitterLogin2Factor(string url){
         }
 
         public static string? GetFileNameFromUrl(string url){
-            string file = Path.GetFileName(new Uri(url).AbsolutePath);
+            return StringUtils.NullIfEmpty(Path.GetFileName(new Uri(url).AbsolutePath));
-            return string.IsNullOrEmpty(file) ? null : file;
         }
 
         public static string GetMediaLink(string url, ImageQuality quality){

lib/TweetLib.Core/Utils/StringUtils.cs

@@ -6,6 +6,10 @@ namespace TweetLib.Core.Utils{
     public static class StringUtils{
         public static readonly string[] EmptyArray = new string[0];
 
+        public static string? NullIfEmpty(string str){
+            return string.IsNullOrEmpty(str) ? null : str;
+        }
+
         public static (string before, string after)? SplitInTwo(string str, char search, int startIndex = 0){
             int index = str.IndexOf(search, startIndex);