Add @Nullable to equalTo value arg

sdk/testing/src/main/java/io/opentelemetry/sdk/testing/assertj/OpenTelemetryAssertions.java

@@ -240,7 +240,7 @@ public static AttributeAssertion satisfies(
    * Returns an {@link AttributeAssertion} that asserts the given {@code key} is present with the
    * given {@code value}.
    */
-  public static <T> AttributeAssertion equalTo(AttributeKey<T> key, T value) {
+  public static <T> AttributeAssertion equalTo(AttributeKey<T> key, @Nullable T value) {
     return AttributeAssertion.create(key, val -> val.isEqualTo(value));
   }
 

sdk/testing/src/test/java/io/opentelemetry/sdk/testing/assertj/AssertUtilTest.java

@@ -59,6 +59,13 @@ void assertAttributesShouldNotThrowIfAllAttributesMatch() {
     AssertUtil.assertAttributes(ATTRIBUTES, assertions);
   }
 
+  @Test
+  void assertAttributesShouldNotThrowIfEqualToNullAndAttributeNotSet() {
+    List<AttributeAssertion> assertions = Arrays.asList(equalTo(WARM, null));
+
+    AssertUtil.assertAttributes(Attributes.empty(), assertions);
+  }
+
   @Test
   void assertAttributesExactlyShouldThrowIfNoAttributeMatch() {
     List<AttributeAssertion> assertions = Arrays.asList(equalTo(WARM, false));