Fix bwc

Author: kanoshiou

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

@@ -638,6 +638,7 @@ foo:keyword
 
 
 evalAfterAvgUsingSameName
+required_capability: remove_empty_attribute_in_merging_output
 from employees 
 | stats avg = avg(salary) 
 | eval avg = 12
@@ -648,6 +649,7 @@ avg:integer
 ;
 
 evalAfterStatsUsingSameName
+required_capability: remove_empty_attribute_in_merging_output
 from employees 
 | stats count = count(emp_no), median = median(salary), top_salaries = TOP(salary, 3, "desc")
 | keep median, top_salaries
@@ -660,6 +662,7 @@ median:integer
 ;
 
 evalAfterStatsUsingSameName2
+required_capability: remove_empty_attribute_in_merging_output
 ROW foo = [10, 11, 12]
 | mv_expand foo
 | stats sum = sum(foo), max = max(foo)

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlCapabilities.java

@@ -413,6 +413,12 @@ public enum Cap {
          */
         RENAME_SEQUENTIAL_PROCESSING,
 
+        /**
+         * Support for removing empty attribute in merging output.
+         * See <a href="https://github.com/elastic/elasticsearch/issues/126392"> ESQL: EVAL after STATS produces an empty column #126392 </a>
+         */
+        REMOVE_EMPTY_ATTRIBUTE_IN_MERGING_OUTPUT,
+
         /**
          * Fix for union-types when some indexes are missing the required field. Done in #111932.
          */