Check for an empty string for the TimeZoneDirectory environment variable (#112831)

* Check for an empty string for the TimeZoneDirectory environment variable

* Code feedback

Author: JongHeonChoi

src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.Unix.NonAndroid.cs

@@ -613,7 +613,7 @@ private static string GetTimeZoneDirectory()
         {
             string? tzDirectory = Environment.GetEnvironmentVariable(TimeZoneDirectoryEnvironmentVariable);
 
-            if (tzDirectory == null)
+            if (string.IsNullOrEmpty(tzDirectory))
             {
                 tzDirectory = DefaultTimeZoneDirectory;
             }