Fix NPE when minBound/maxBound is not set before being called. (#3605)

Signed-off-by: George Apaaboah <george.apaaboah@gmail.com>

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

@@ -227,12 +227,12 @@ public HistogramAggregationBuilder offset(double offset) {
 
     /** Get the current minimum bound that is set on this builder. */
     public double minBound() {
-        return extendedBounds.getMin();
+        return DoubleBounds.getEffectiveMin(extendedBounds);
     }
 
     /** Get the current maximum bound that is set on this builder. */
     public double maxBound() {
-        return extendedBounds.getMax();
+        return DoubleBounds.getEffectiveMax(extendedBounds);
     }
 
     protected DoubleBounds extendedBounds() {

server/src/test/java/org/opensearch/search/aggregations/bucket/HistogramTests.java

@@ -103,6 +103,19 @@ public void testInvalidBounds() {
         assertThat(ex.getMessage(), equalTo("max bound [0.4] must be greater than min bound [0.5]"));
     }
 
+    /**
+     * Check that minBound/maxBound does not throw {@link NullPointerException} when called before being set.
+     */
+    public void testMinBoundMaxBoundDefaultValues() {
+        HistogramAggregationBuilder factory = new HistogramAggregationBuilder("foo");
+
+        double minValue = factory.minBound();
+        double maxValue = factory.maxBound();
+
+        assertThat(minValue, equalTo(Double.POSITIVE_INFINITY));
+        assertThat(maxValue, equalTo(Double.NEGATIVE_INFINITY));
+    }
+
     private List<BucketOrder> randomOrder() {
         List<BucketOrder> orders = new ArrayList<>();
         switch (randomInt(4)) {