Describe the bug
A NullPointerException occurs when the methods minBound() and maxBound

To Reproduce
A unit test that does not set the min and max values, example for the minBound:

public void testNPE() {
        HistogramAggregationBuilder factory = new HistogramAggregationBuilder("foo");

        try {
            factory.minBound();
        }
        catch (Exception e) {
            assertThat(e, instanceOf(NullPointerException.class));
        }
    }

Expected behavior
It should return null

Host/Environment (please complete the following information):

• Java version : 11
• OS: Ubuntu
• Version 20.04.4 LTS