Simplify httpClient validation logic

Use null defaults for connectTimeout and proxy in Builder, making
validation cleaner by simply checking for null values. Set defaults
only when no custom HttpClient is provided. Remove unnecessary comments.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: horgh

src/main/java/com/maxmind/geoip2/WebServiceClient.java

@@ -141,7 +141,6 @@ private WebServiceClient(Builder builder) {
 
         requestTimeout = builder.requestTimeout;
 
-        // Use custom HttpClient if provided, otherwise create a default one
         if (builder.httpClient != null) {
             httpClient = builder.httpClient;
         } else {
@@ -177,11 +176,11 @@ public static final class Builder {
         int port = 443;
         boolean useHttps = true;
 
-        Duration connectTimeout = Duration.ofSeconds(3);
+        Duration connectTimeout = null;
         Duration requestTimeout = Duration.ofSeconds(20);
 
         List<String> locales = Collections.singletonList("en");
-        private ProxySelector proxy = ProxySelector.getDefault();
+        private ProxySelector proxy = null;
         private HttpClient httpClient = null;
 
         /**
@@ -322,18 +321,23 @@ public Builder httpClient(HttpClient val) {
          */
         public WebServiceClient build() {
             if (httpClient != null) {
-                // Check if connectTimeout was changed from default
-                if (!connectTimeout.equals(Duration.ofSeconds(3))) {
+                if (connectTimeout != null) {
                     throw new IllegalArgumentException(
                         "Cannot set both httpClient and connectTimeout. "
                         + "Configure timeout on the provided HttpClient instead.");
                 }
-                // Check if proxy was changed from default
-                if (proxy != ProxySelector.getDefault()) {
+                if (proxy != null) {
                     throw new IllegalArgumentException(
                         "Cannot set both httpClient and proxy. "
                         + "Configure proxy on the provided HttpClient instead.");
                 }
+            } else {
+                if (connectTimeout == null) {
+                    connectTimeout = Duration.ofSeconds(3);
+                }
+                if (proxy == null) {
+                    proxy = ProxySelector.getDefault();
+                }
             }
             return new WebServiceClient(this);
         }