refactor(dpp): use if-let instead of unwrap for validation error extraction

Replace is_valid() check + unwrap() with if-let pattern to eliminate
fragile coupling to ValidationResult internals.

Addresses CodeRabbit nitpick on PR #3138.

Author: thepastaclaw

packages/rs-dpp/src/state_transition/state_transitions/identity/identity_topup_transition/v0/v0_methods.rs

@@ -37,8 +37,7 @@ impl IdentityTopUpTransitionMethodsV0 for IdentityTopUpTransitionV0 {
         #[cfg(feature = "validation")]
         {
             let validation_result = asset_lock_proof.validate_structure(platform_version)?;
-            if !validation_result.is_valid() {
-                let first_error = validation_result.errors.into_iter().next().unwrap();
+            if let Some(first_error) = validation_result.errors.into_iter().next() {
                 return Err(ProtocolError::ConsensusError(Box::new(first_error)));
             }
         }