Remove redundant checks in assertValidName, mark param @nullable

- Mark assertValidName parameter as @nullable since callers may pass null
- Keep null check in assertValidName (required for @nullable contract)
- Remove redundant empty check that was duplicated in isValidName
- Use String.valueOf() for null-safe error message formatting

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: andimarek

src/main/java/graphql/Assert.java

@@ -231,11 +231,11 @@ public static void assertFalse(boolean condition, String msgFmt, Object arg1, Ob
      *
      * @return the name if valid, or AssertException if invalid.
      */
-    public static String assertValidName(String name) {
-        if (name != null && !name.isEmpty() && isValidName(name)) {
+    public static String assertValidName(@Nullable String name) {
+        if (name != null && isValidName(name)) {
             return name;
         }
-        return throwAssert(invalidNameErrorMessage, name);
+        return throwAssert(invalidNameErrorMessage, String.valueOf(name));
     }
 
     /**