#201 - return not null if no env is present

Author: Stepami

src/Domain/HydraScript.Domain.BackEnd/Impl/Frames/EnvFrame.cs

@@ -4,7 +4,7 @@ public sealed class EnvFrame(IEnvironmentVariableProvider provider) : IFrame
 {
     public object? this[string id]
     {
-        get => provider.GetEnvironmentVariable(id);
+        get => provider.GetEnvironmentVariable(id) ?? string.Empty;
         set => provider.SetEnvironmentVariable(id, value?.ToString());
     }
 }