Make method name optional in X::Invalid::ComputedValue

Sometimes we don't know what the user-facing method was used.

src/core/Exception.pm

@@ -1280,7 +1280,8 @@ my class X::Invalid::ComputedValue is Exception {
     has $.value;
     has $.reason;
     method message {
-        "$.name on $.method computed to $.value, which cannot be used"
+        "$.name {"on $.method " if $.method}computed to $.value,"
+            ~ " which cannot be used"
             ~ (" because $.reason" if $.reason);
     }
 }