Fix bwc tests introducing capability

Author: carlosdelest

x-pack/plugin/esql/qa/testFixtures/src/main/resources/vector-cosine-similarity.csv-spec

@@ -79,6 +79,7 @@ avg:double | min:double | max:double
 
 similarityWithNull
 required_capability: cosine_vector_similarity_function
+required_capability: vector_similarity_functions_support_null
 
 from colors
 | eval similarity = v_cosine(rgb_vector, null) 

x-pack/plugin/esql/qa/testFixtures/src/main/resources/vector-dot-product.csv-spec

@@ -76,6 +76,7 @@ avg:double | min:double | max:double
 
 similarityWithNull
 required_capability: dot_product_vector_similarity_function
+required_capability: vector_similarity_functions_support_null
 
 from colors
 | eval similarity = v_dot_product(rgb_vector, null) 

x-pack/plugin/esql/qa/testFixtures/src/main/resources/vector-l1-norm.csv-spec

@@ -76,6 +76,7 @@ avg:double | min:double | max:double
 
 similarityWithNull
 required_capability: l1_norm_vector_similarity_function
+required_capability: vector_similarity_functions_support_null
 
 from colors
 | eval similarity = v_l1_norm(rgb_vector, null) 

x-pack/plugin/esql/qa/testFixtures/src/main/resources/vector-l2-norm.csv-spec

@@ -76,6 +76,7 @@ avg:double | min:double | max:double
 
 similarityWithNull
 required_capability: l2_norm_vector_similarity_function
+required_capability: vector_similarity_functions_support_null
 
 from colors
 | eval similarity = v_l2_norm(rgb_vector, null) 

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

@@ -1345,7 +1345,11 @@ public enum Cap {
         /**
          * Support correct counting of skipped shards.
          */
-        CORRECT_SKIPPED_SHARDS_COUNT;
+        CORRECT_SKIPPED_SHARDS_COUNT,
+        /**
+         * Support correct counting of skipped shards.
+         */
+        VECTOR_SIMILARITY_FUNCTIONS_SUPPORT_NULL;
 
         private final boolean enabled;