fix: Do not throw null reference exception accessing a missing item. (#300)

Signed-off-by: Ryan Lamb <[email protected]>

Author: kinyoklion

src/main/java/dev/openfeature/sdk/ImmutableStructure.java

@@ -50,7 +50,7 @@ public Set<String> keySet() {
     @Override
     public Value getValue(String key) {
         Value value = this.attributes.get(key);
-        return value.clone();
+        return value != null ? value.clone() : null;
     }
 
     /**

src/test/java/dev/openfeature/sdk/ImmutableStructureTest.java

@@ -10,11 +10,7 @@
 import java.util.Map;
 import java.util.Set;
 
-import static org.junit.jupiter.api.Assertions.assertArrayEquals;
-import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.junit.jupiter.api.Assertions.assertNotEquals;
-import static org.junit.jupiter.api.Assertions.assertNotSame;
-import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.*;
 
 class ImmutableStructureTest {
     @Test void noArgShouldContainEmptyAttributes() {
@@ -108,4 +104,11 @@ void ModifyingTheValuesReturnByTheKeySetMethodShouldNotModifyTheUnderlyingImmuta
         keys.remove("key1");
         assertEquals(2, structure.keySet().size());
     }
+
+    @Test
+    void GettingAMissingValueShouldReturnNull() {
+        ImmutableStructure structure = new ImmutableStructure();
+        Object value = structure.getValue("missing");
+        assertNull(value);
+    }
 }