Deduplicate returnVal assignment and return logic for Char.IsDigit overloads

Co-authored-by: YuliiaKovalova <95473390+YuliiaKovalova@users.noreply.github.com>

Author: Copilot

src/Build/Evaluation/Expander/WellKnownFunctions.cs

@@ -879,15 +879,21 @@ internal static bool TryExecuteWellKnownFunction(string methodName, Type receive
                 {
                     if (string.Equals(methodName, nameof(char.IsDigit), StringComparison.OrdinalIgnoreCase))
                     {
+                        bool? result = null;
+                        
                         if (ParseArgs.TryGetArg(args, out string? arg0) && arg0?.Length == 1)
                         {
                             char c = arg0[0];
-                            returnVal = char.IsDigit(c);
-                            return true;
+                            result = char.IsDigit(c);
                         }
                         else if (ParseArgs.TryGetArgs(args, out string? str, out int index) && str != null)
                         {
-                            returnVal = char.IsDigit(str, index);
+                            result = char.IsDigit(str, index);
+                        }
+                        
+                        if (result.HasValue)
+                        {
+                            returnVal = result.Value;
                             return true;
                         }
                     }