Add missing test case

Author: JonasKunz

x-pack/plugin/mapper-exponential-histogram/src/test/java/org/elasticsearch/xpack/exponentialhistogram/ExponentialHistogramFieldMapperTests.java

@@ -389,7 +389,15 @@ protected List<ExampleMalformedValue> exampleMalformedValues() {
                     .endArray()
                     .endObject()
                     .endObject()
-            ).errorMatches("has a total value count exceeding the allowed maximum value of " + Long.MAX_VALUE)
+            ).errorMatches("has a total value count exceeding the allowed maximum value of " + Long.MAX_VALUE),
+
+            // Non-Zero sum for empty histogram
+            exampleMalformedValue(
+                b -> b.startObject()
+                    .field("scale", 0)
+                    .field("sum", 42.0)
+                    .endObject()
+            ).errorMatches("sum field must be zero if the histogram is empty, but got 42.0")
         );
     }