authbase: gracefully handle no-default UI helper case

If there is no hard-coded UI helper, we should gracefully handle the
lookup by returning false, rather than crashing due to a argument
exception when passing `null` or an empty string to a Path API.

Author: mjcheetham

src/shared/Core/Authentication/AuthenticationBase.cs

@@ -140,6 +140,11 @@ protected bool TryFindHelperCommand(string envar, string configName, string defa
 
                 Context.Trace.WriteLine($"UI helper override specified: '{helperName}'.");
             }
+            else if (string.IsNullOrWhiteSpace(defaultValue))
+            {
+                Context.Trace.WriteLine("No default UI supplied.");
+                return false;
+            }
             else
             {
                 Context.Trace.WriteLine($"Using default UI helper: '{defaultValue}'.");