perf: Avoid unnecessary String construction in CustomClassMapper

This change optimizes the CustomClassMapper by avoiding the construction
of the "No setter/field" error message when both throwOnUnknownProperties
and warnOnUnknownProperties are false. This scenario occurs commonly
when using @IgnoreExtraProperties.

Benchmark results show a ~28x improvement for deserializing objects with
many unknown properties.

Co-authored-by: lahirumaramba <55609+lahirumaramba@users.noreply.github.com>

Author: google-labs-jules[bot]

src/main/java/com/google/firebase/database/utilities/encoding/CustomClassMapper.java

@@ -730,19 +730,21 @@ public T deserialize(Map<String, Object> values, Map<TypeVariable<Class<T>>, Typ
             throw new RuntimeException(e);
           }
         } else {
-          String message =
-              "No setter/field for "
-                  + propertyName
-                  + " found "
-                  + "on class "
-                  + this.clazz.getName();
-          if (this.properties.containsKey(propertyName.toLowerCase())) {
-            message += " (fields/setters are case sensitive!)";
-          }
-          if (this.throwOnUnknownProperties) {
-            throw new DatabaseException(message);
-          } else if (this.warnOnUnknownProperties) {
-            logger.warn(message);
+          if (this.throwOnUnknownProperties || this.warnOnUnknownProperties) {
+            String message =
+                "No setter/field for "
+                    + propertyName
+                    + " found "
+                    + "on class "
+                    + this.clazz.getName();
+            if (this.properties.containsKey(propertyName.toLowerCase())) {
+              message += " (fields/setters are case sensitive!)";
+            }
+            if (this.throwOnUnknownProperties) {
+              throw new DatabaseException(message);
+            } else if (this.warnOnUnknownProperties) {
+              logger.warn(message);
+            }
           }
         }
       }