Select: fix null-valued options (#6493)

Author: stsrki

Source/Blazorise/Components/Select/SelectItem.razor.cs

@@ -52,7 +52,7 @@ public bool CompareTo( object value )
     /// <summary>
     /// Convert the value to string because option tags are working with string internally. Otherwise some datatypes like booleans will not work as expected.
     /// </summary>
-    protected string StringValue => Value?.ToString();
+    protected string StringValue => Value?.ToString() ?? string.Empty;
 
     /// <summary>
     /// Gets or sets the item value.