Add sanity checks for extension keys

Author: malczuuu

src/main/java/io/github/malczuuu/problem4j/core/Problem.java

@@ -42,11 +42,15 @@ static ProblemBuilder builder() {
   /**
    * Creates a named extension for use in a {@link Problem}.
    *
-   * @param key the extension key
+   * @param key the extension key, must not be {@code null}
    * @param value the extension value
    * @return a new {@link Extension} instance
+   * @throws IllegalArgumentException if the {@code key} is {@code null}
    */
   static Extension extension(String key, Object value) {
+    if (key == null) {
+      throw new IllegalArgumentException("key cannot be null");
+    }
     return new ProblemImpl.ExtensionImpl(key, value);
   }
 

src/test/java/io/github/malczuuu/problem4j/core/ProblemTests.java

@@ -1,6 +1,7 @@
 package io.github.malczuuu.problem4j.core;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 import java.net.URI;
 import java.util.Arrays;
@@ -48,4 +49,19 @@ void givenProblem_whenToBuilder_shouldBeAbleToRecreateOriginal() {
     assertThat(problem).isNotSameAs(copy);
     assertThat(problem).isEqualTo(copy);
   }
+
+  @Test
+  void givenProblemExtensionWithNullKey_shouldThrowIllegalArgumentException() {
+    assertThatThrownBy(() -> Problem.extension(null, "v"))
+        .isInstanceOf(IllegalArgumentException.class)
+        .hasMessage("key cannot be null");
+  }
+
+  @Test
+  void givenProblemExtensionWithKey_shouldCreateExtension() {
+    Problem.Extension ext = Problem.extension("code", 123);
+
+    assertThat(ext.getKey()).isEqualTo("code");
+    assertThat(ext.getValue()).isEqualTo(123);
+  }
 }