Remove null test case as it seems to be broken on ingestion

Author: JonasKunz

x-pack/plugin/esql/qa/testFixtures/src/main/resources/data/exp_histo_sample.csv

@@ -1,6 +1,5 @@
 name:keyword,histo:exponential_histogram
 empty,{"scale":-7}
-null_histo,null
 full,{"scale":12\,"zero":{"threshold":0.123456\,"count":42}\,"positive":{"indices":[-1000000\,-10\,25\,26\,99999999]\,"counts":[2\,1\,3\,4\,5]}\,"negative":{"indices":[-1000\,123456\,-999]\,"counts":[10\,11\,20]}}
 no_zero_bucket,{"scale":-10\,"positive":{"indices":[1\,2\,3\,4\,5]\,"counts":[6\,7\,8\,9\,10]}\,"negative":{"indices":[-1\,0\,1\,2\,3\,4\,5\,6\,7\,8]\,"counts":[1\,2\,3\,4\,5\,6\,7\,8\,9\,10]}}
 positive_only,{"scale":0\,"positive":{"indices":[-100\,10\,20]\,"counts":[3\,2\,1]}}

x-pack/plugin/esql/qa/testFixtures/src/main/resources/exponential_histogram.csv-spec

@@ -5,7 +5,6 @@ FROM exp_histo_sample | KEEP histo
 
 histo:exponential_histogram
 {"scale":-7}
-null
 {"scale":12}
 {"scale":-10}
 {"scale":0}