code: public @NotNull ThrowableAssert hasNoCause() {
      isNotNull();
      Throwable actualCause = actual.getCause();
      if (actualCause == null) {
        return this;
      }
      failIfCustomMessageIsSet();
      throw failure(format("expected exception without cause, but cause was:<%s>", actualCause.getClass()));
    }
docs: Verifies that the actual {@code Throwable} does not have a cause.
  
  @return this assertion object.
  @throws AssertionError if the actual {@code Throwable} is {@code null}.
  @throws AssertionError if the actual {@code Throwable} has a cause.