Add @Nullable to equalTo value arg

Author: trask

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));
   }