Fix null-ref

Author: javiercn

src/StaticWebAssetsSdk/Tasks/Data/ContentTypeProvider.cs

@@ -474,9 +474,6 @@ private bool TryGetMapping(StaticWebAssetGlobMatcher.MatchContext context, TaskL
 
 #if NET9_0_OR_GREATER
     private static ReadOnlySpan<char> ResolvePathWithoutCompressedExtension(ReadOnlySpan<char> fileName, out bool hasCompressedExtension)
-#else
-    private static string ResolvePathWithoutCompressedExtension(string fileName, out bool hasCompressedExtension)
-#endif
     {
         var extension = Path.GetExtension(fileName);
         hasCompressedExtension = extension.Equals(".gz", StringComparison.OrdinalIgnoreCase) || extension.Equals(".br", StringComparison.OrdinalIgnoreCase);
@@ -485,14 +482,27 @@ private static string ResolvePathWithoutCompressedExtension(string fileName, out
             var fileNameNoExtension = Path.GetFileNameWithoutExtension(fileName);
             if (!Path.GetExtension(fileNameNoExtension).Equals("", StringComparison.Ordinal))
             {
-#if NET9_0_OR_GREATER
                 return fileName[..fileNameNoExtension.Length];
+            }
+        }
+
+        return fileName;
+    }
 #else
+    private static string ResolvePathWithoutCompressedExtension(string fileName, out bool hasCompressedExtension)
+    {
+        var extension = Path.GetExtension(fileName);
+        hasCompressedExtension = extension != null && extension.Equals(".gz", StringComparison.OrdinalIgnoreCase) || extension.Equals(".br", StringComparison.OrdinalIgnoreCase);
+        if (hasCompressedExtension)
+        {
+            var fileNameNoExtension = Path.GetFileNameWithoutExtension(fileName);
+            if (!string.Equals(Path.GetExtension(fileNameNoExtension), "", StringComparison.Ordinal))
+            {
                 return fileName.Substring(0, fileNameNoExtension.Length);
-#endif
             }
         }
 
         return fileName;
     }
+#endif
 }