fix: add nullability operator to variable splitting to prevent potential null reference exceptions

Author: thomasduft

src/testr.Cli/Domain/VariablesHelper.cs

@@ -13,7 +13,7 @@ public static Dictionary<string, string> CreateVariables(
     }
 
     return variables
-      .Select(v => v.Split('='))
+      .Select(v => v!.Split('='))
       .Where(parts => parts.Length == 2)
       .ToDictionary(parts => parts[0].Trim(), parts => parts[1].Trim());
   }