fix ClientOverrideConfiguration.Builder empty headers throwing NPE (#4726)

 fix ClientOverrideConfiguration.Builder empty headers throwing NullPointerException

Author: L-Applin

core/sdk-core/src/main/java/software/amazon/awssdk/core/client/config/ClientOverrideConfiguration.java

@@ -705,7 +705,9 @@ public void setHeaders(Map<String, List<String>> additionalHttpHeaders) {
 
         @Override
         public Map<String, List<String>> headers() {
-            return CollectionUtils.unmodifiableMapOfLists(config.option(ADDITIONAL_HTTP_HEADERS));
+            Map<String, List<String>> option = Validate
+                .getOrDefault(config.option(ADDITIONAL_HTTP_HEADERS), Collections::emptyMap);
+            return CollectionUtils.unmodifiableMapOfLists(option);
         }
 
         @Override

core/sdk-core/src/test/java/software/amazon/awssdk/core/client/config/ClientOverrideConfigurationTest.java

@@ -16,6 +16,7 @@
 package software.amazon.awssdk.core.client.config;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.junit.jupiter.api.Assertions.fail;
@@ -278,4 +279,17 @@ public void executionAttributes_listPreviouslyAdded_appendedToList() {
         ClientOverrideConfiguration overrideConfig = builder.build();
         assertThat(overrideConfig.executionAttributes().getAttribute(addedAttribute)).isEqualTo(addedValue);
     }
+
+    @Test
+    void builder_headersWithEmptyAdditionalHeaders_shouldReturnEmptyMap_shouldNotThrowNullPointerException() {
+        ClientOverrideConfiguration.Builder builder = ClientOverrideConfiguration.builder();
+        Map<String, List<String>> headers = builder.headers();
+        assertDoesNotThrow(() -> headers.isEmpty());
+        assertThat(headers).isNotNull();
+        Map<String, List<String>> newHeaders = new HashMap<>();
+        newHeaders.put("my-key", Collections.singletonList("my-value"));
+        assertDoesNotThrow(() -> newHeaders.putAll(headers));
+        assertThat(newHeaders).containsEntry("my-key", Collections.singletonList("my-value"))
+            .hasSize(1);
+    }
 }

utils/src/main/java/software/amazon/awssdk/utils/UnmodifiableMapOfLists.java

@@ -40,7 +40,7 @@ class UnmodifiableMapOfLists<T, U> implements Map<T, List<U>>, Serializable {
     private final Map<T, List<U>> delegate;
 
     UnmodifiableMapOfLists(Map<T, List<U>> delegate) {
-        this.delegate = delegate;
+        this.delegate = Validate.paramNotNull(delegate, "delegate");
     }
 
     @Override