Fix NPE when minBound/maxBound is not set before being called.

[GeorgeAp]

Description

Resolves a NullPointerException which occurs when the methods minBound() and maxBound

OpenSearch/server/src/main/java/org/opensearch/search/aggregations/bucket/histogram/HistogramAggregationBuilder.java

Lines 228 to 236 in 8844f05

	/** Get the current minimum bound that is set on this builder. */
	public double minBound() {
	return extendedBounds.getMin();
	}
	/** Get the current maximum bound that is set on this builder. */
	public double maxBound() {
	return extendedBounds.getMax();
	}

are called before they are set in DoubleBounds

OpenSearch/server/src/main/java/org/opensearch/search/aggregations/bucket/histogram/DoubleBounds.java

Lines 154 to 160 in 8844f05

	public Double getMin() {
	return min;
	}
	public Double getMax() {
	return max;
	}

Issues Resolved

#3604

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[opensearch-ci-bot]

✅   Gradle Check success 08956a0
Log 6068

Reports 6068

[dblock]

Thanks for fixing this @GeorgeAp, @reta feel free to merge if your concerns are answered!